diff --git a/Source/JavaScriptCore/ChangeLog b/Source/JavaScriptCore/ChangeLog
index 64b553e..ac6f664 100644
--- a/Source/JavaScriptCore/ChangeLog
+++ b/Source/JavaScriptCore/ChangeLog
@@ -1,3 +1,13 @@
+2019-01-27  Chris Fleizach  <cfleizach@apple.com>
+
+        AX: Introduce a static accessibility tree
+        https://bugs.webkit.org/show_bug.cgi?id=193348
+        <rdar://problem/47203295>
+
+        Reviewed by Ryosuke Niwa.
+
+        * Configurations/FeatureDefines.xcconfig:
+
 2019-01-26  Devin Rousso  <drousso@apple.com>
 
         Web Inspector: provide a way to edit the user agent of a remote target
diff --git a/Source/JavaScriptCore/Configurations/FeatureDefines.xcconfig b/Source/JavaScriptCore/Configurations/FeatureDefines.xcconfig
index 9122a25..230623e 100644
--- a/Source/JavaScriptCore/Configurations/FeatureDefines.xcconfig
+++ b/Source/JavaScriptCore/Configurations/FeatureDefines.xcconfig
@@ -40,6 +40,8 @@
 ENABLE_ACCELERATED_OVERFLOW_SCROLLING = $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING_$(WK_COCOA_TOUCH));
 ENABLE_ACCELERATED_OVERFLOW_SCROLLING_cocoatouch = ENABLE_ACCELERATED_OVERFLOW_SCROLLING;
 
+ENABLE_ACCESSIBILITY_ISOLATED_TREE = ;
+
 ENABLE_APPLE_PAY = $(ENABLE_APPLE_PAY_$(WK_PLATFORM_NAME));
 ENABLE_APPLE_PAY_iphoneos = ENABLE_APPLE_PAY;
 ENABLE_APPLE_PAY_iphonesimulator = ENABLE_APPLE_PAY;
@@ -407,4 +409,4 @@
 
 ENABLE_XSLT = ENABLE_XSLT;
 
-FEATURE_DEFINES = $(ENABLE_3D_TRANSFORMS) $(ENABLE_ACCELERATED_2D_CANVAS) $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING) $(ENABLE_APPLE_PAY) $(ENABLE_APPLE_PAY_SESSION_V3) $(ENABLE_APPLE_PAY_SESSION_V4) $(ENABLE_APPLICATION_MANIFEST) $(ENABLE_ATTACHMENT_ELEMENT) $(ENABLE_AVF_CAPTIONS) $(ENABLE_C_LOOP) $(ENABLE_CACHE_PARTITIONING) $(ENABLE_CHANNEL_MESSAGING) $(ENABLE_CONTENT_FILTERING) $(ENABLE_CSS3_TEXT) $(ENABLE_CSS_BOX_DECORATION_BREAK) $(ENABLE_CSS_COMPOSITING) $(ENABLE_CSS_DEVICE_ADAPTATION) $(ENABLE_CSS_IMAGE_ORIENTATION) $(ENABLE_CSS_IMAGE_RESOLUTION) $(ENABLE_CSS_PAINTING_API) $(ENABLE_CSS_SCROLL_SNAP) $(ENABLE_CSS_SELECTORS_LEVEL4) $(ENABLE_CSS_TRAILING_WORD) $(ENABLE_CSS_TYPED_OM) $(ENABLE_CURSOR_VISIBILITY) $(ENABLE_CUSTOM_SCHEME_HANDLER) $(ENABLE_CSS_CONIC_GRADIENTS) $(ENABLE_DARK_MODE_CSS) $(ENABLE_DASHBOARD_SUPPORT) $(ENABLE_DATA_INTERACTION) $(ENABLE_DATACUE_VALUE) $(ENABLE_DATALIST_ELEMENT) $(ENABLE_DEVICE_ORIENTATION) $(ENABLE_DRAG_SUPPORT) $(ENABLE_ENCRYPTED_MEDIA) $(ENABLE_EXPERIMENTAL_FEATURES) $(ENABLE_FAST_JIT_PERMISSIONS) $(ENABLE_FILTERS_LEVEL_2) $(ENABLE_FTL_JIT) $(ENABLE_FULLSCREEN_API) $(ENABLE_GAMEPAD) $(ENABLE_GEOLOCATION) $(ENABLE_ICONDATABASE) $(ENABLE_INDEXED_DATABASE) $(ENABLE_INDEXED_DATABASE_IN_WORKERS) $(ENABLE_INPUT_TYPE_COLOR) $(ENABLE_INPUT_TYPE_DATE) $(ENABLE_INPUT_TYPE_DATETIME_INCOMPLETE) $(ENABLE_INPUT_TYPE_DATETIMELOCAL) $(ENABLE_INPUT_TYPE_MONTH) $(ENABLE_INPUT_TYPE_TIME) $(ENABLE_INPUT_TYPE_WEEK) $(ENABLE_INTERSECTION_OBSERVER) $(ENABLE_INTL) $(ENABLE_IOS_GESTURE_EVENTS) $(ENABLE_IOS_TOUCH_EVENTS) $(ENABLE_JIT) $(ENABLE_KEYBOARD_CODE_ATTRIBUTE) $(ENABLE_KEYBOARD_KEY_ATTRIBUTE) $(ENABLE_LAYOUT_FORMATTING_CONTEXT) $(ENABLE_LEGACY_CSS_VENDOR_PREFIXES) $(ENABLE_LEGACY_CUSTOM_PROTOCOL_MANAGER) $(ENABLE_LEGACY_ENCRYPTED_MEDIA) $(ENABLE_LETTERPRESS) $(ENABLE_MAC_GESTURE_EVENTS) $(ENABLE_MAC_VIDEO_TOOLBOX) $(ENABLE_MATHML) $(ENABLE_MEDIA_CAPTURE) $(ENABLE_MEDIA_CONTROLS_SCRIPT) $(ENABLE_MEDIA_SESSION) $(ENABLE_MEDIA_SOURCE) $(ENABLE_MEDIA_STATISTICS) $(ENABLE_MEDIA_STREAM) $(ENABLE_METER_ELEMENT) $(ENABLE_MHTML) $(ENABLE_MOUSE_CURSOR_SCALE) $(ENABLE_NAVIGATOR_CONTENT_UTILS) $(ENABLE_NAVIGATOR_STANDALONE) $(ENABLE_NOTIFICATIONS) $(ENABLE_PAYMENT_REQUEST) $(ENABLE_PDFKIT_PLUGIN) $(ENABLE_POINTER_EVENTS) $(ENABLE_POINTER_LOCK) $(ENABLE_PUBLIC_SUFFIX_LIST) $(ENABLE_QUOTA) $(ENABLE_REMOTE_INSPECTOR) $(ENABLE_RESOLUTION_MEDIA_QUERY) $(ENABLE_RESOURCE_LOAD_STATISTICS) $(ENABLE_RESOURCE_USAGE) $(ENABLE_RUBBER_BANDING) $(ENABLE_SERVICE_CONTROLS) $(ENABLE_SERVICE_WORKER) $(ENABLE_SPEECH_SYNTHESIS) $(ENABLE_STREAMS_API) $(ENABLE_WEB_CRYPTO) $(ENABLE_SVG_FONTS) $(ENABLE_TELEPHONE_NUMBER_DETECTION) $(ENABLE_TEXT_AUTOSIZING) $(ENABLE_TOUCH_EVENTS) $(ENABLE_USER_MESSAGE_HANDLERS) $(ENABLE_USERSELECT_ALL) $(ENABLE_VARIATION_FONTS) $(ENABLE_VIDEO) $(ENABLE_VIDEO_PRESENTATION_MODE) $(ENABLE_VIDEO_TRACK) $(ENABLE_VIDEO_USES_ELEMENT_FULLSCREEN) $(ENABLE_WEB_API_STATISTICS) $(ENABLE_WEB_AUDIO) $(ENABLE_WEB_AUTHN) $(ENABLE_WEB_RTC) $(ENABLE_WEBASSEMBLY) $(ENABLE_WEBASSEMBLY_STREAMING_API) $(ENABLE_WEBGL) $(ENABLE_WEBGL2) $(ENABLE_WEBGPU) $(ENABLE_WEBMETAL) $(ENABLE_WIRELESS_PLAYBACK_TARGET) $(ENABLE_XSLT);
+FEATURE_DEFINES = $(ENABLE_3D_TRANSFORMS) $(ENABLE_ACCELERATED_2D_CANVAS) $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING) $(ENABLE_ACCESSIBILITY_ISOLATED_TREE) $(ENABLE_APPLE_PAY) $(ENABLE_APPLE_PAY_SESSION_V3) $(ENABLE_APPLE_PAY_SESSION_V4) $(ENABLE_APPLICATION_MANIFEST) $(ENABLE_ATTACHMENT_ELEMENT) $(ENABLE_AVF_CAPTIONS) $(ENABLE_C_LOOP) $(ENABLE_CACHE_PARTITIONING) $(ENABLE_CHANNEL_MESSAGING) $(ENABLE_CONTENT_FILTERING) $(ENABLE_CSS3_TEXT) $(ENABLE_CSS_BOX_DECORATION_BREAK) $(ENABLE_CSS_COMPOSITING) $(ENABLE_CSS_DEVICE_ADAPTATION) $(ENABLE_CSS_IMAGE_ORIENTATION) $(ENABLE_CSS_IMAGE_RESOLUTION) $(ENABLE_CSS_PAINTING_API) $(ENABLE_CSS_SCROLL_SNAP) $(ENABLE_CSS_SELECTORS_LEVEL4) $(ENABLE_CSS_TRAILING_WORD) $(ENABLE_CSS_TYPED_OM) $(ENABLE_CURSOR_VISIBILITY) $(ENABLE_CUSTOM_SCHEME_HANDLER) $(ENABLE_CSS_CONIC_GRADIENTS) $(ENABLE_DARK_MODE_CSS) $(ENABLE_DASHBOARD_SUPPORT) $(ENABLE_DATA_INTERACTION) $(ENABLE_DATACUE_VALUE) $(ENABLE_DATALIST_ELEMENT) $(ENABLE_DEVICE_ORIENTATION) $(ENABLE_DRAG_SUPPORT) $(ENABLE_ENCRYPTED_MEDIA) $(ENABLE_EXPERIMENTAL_FEATURES) $(ENABLE_FAST_JIT_PERMISSIONS) $(ENABLE_FILTERS_LEVEL_2) $(ENABLE_FTL_JIT) $(ENABLE_FULLSCREEN_API) $(ENABLE_GAMEPAD) $(ENABLE_GEOLOCATION) $(ENABLE_ICONDATABASE) $(ENABLE_INDEXED_DATABASE) $(ENABLE_INDEXED_DATABASE_IN_WORKERS) $(ENABLE_INPUT_TYPE_COLOR) $(ENABLE_INPUT_TYPE_DATE) $(ENABLE_INPUT_TYPE_DATETIME_INCOMPLETE) $(ENABLE_INPUT_TYPE_DATETIMELOCAL) $(ENABLE_INPUT_TYPE_MONTH) $(ENABLE_INPUT_TYPE_TIME) $(ENABLE_INPUT_TYPE_WEEK) $(ENABLE_INTERSECTION_OBSERVER) $(ENABLE_INTL) $(ENABLE_IOS_GESTURE_EVENTS) $(ENABLE_IOS_TOUCH_EVENTS) $(ENABLE_JIT) $(ENABLE_KEYBOARD_CODE_ATTRIBUTE) $(ENABLE_KEYBOARD_KEY_ATTRIBUTE) $(ENABLE_LAYOUT_FORMATTING_CONTEXT) $(ENABLE_LEGACY_CSS_VENDOR_PREFIXES) $(ENABLE_LEGACY_CUSTOM_PROTOCOL_MANAGER) $(ENABLE_LEGACY_ENCRYPTED_MEDIA) $(ENABLE_LETTERPRESS) $(ENABLE_MAC_GESTURE_EVENTS) $(ENABLE_MAC_VIDEO_TOOLBOX) $(ENABLE_MATHML) $(ENABLE_MEDIA_CAPTURE) $(ENABLE_MEDIA_CONTROLS_SCRIPT) $(ENABLE_MEDIA_SESSION) $(ENABLE_MEDIA_SOURCE) $(ENABLE_MEDIA_STATISTICS) $(ENABLE_MEDIA_STREAM) $(ENABLE_METER_ELEMENT) $(ENABLE_MHTML) $(ENABLE_MOUSE_CURSOR_SCALE) $(ENABLE_NAVIGATOR_CONTENT_UTILS) $(ENABLE_NAVIGATOR_STANDALONE) $(ENABLE_NOTIFICATIONS) $(ENABLE_PAYMENT_REQUEST) $(ENABLE_PDFKIT_PLUGIN) $(ENABLE_POINTER_EVENTS) $(ENABLE_POINTER_LOCK) $(ENABLE_PUBLIC_SUFFIX_LIST) $(ENABLE_QUOTA) $(ENABLE_REMOTE_INSPECTOR) $(ENABLE_RESOLUTION_MEDIA_QUERY) $(ENABLE_RESOURCE_LOAD_STATISTICS) $(ENABLE_RESOURCE_USAGE) $(ENABLE_RUBBER_BANDING) $(ENABLE_SERVICE_CONTROLS) $(ENABLE_SERVICE_WORKER) $(ENABLE_SPEECH_SYNTHESIS) $(ENABLE_STREAMS_API) $(ENABLE_WEB_CRYPTO) $(ENABLE_SVG_FONTS) $(ENABLE_TELEPHONE_NUMBER_DETECTION) $(ENABLE_TEXT_AUTOSIZING) $(ENABLE_TOUCH_EVENTS) $(ENABLE_USER_MESSAGE_HANDLERS) $(ENABLE_USERSELECT_ALL) $(ENABLE_VARIATION_FONTS) $(ENABLE_VIDEO) $(ENABLE_VIDEO_PRESENTATION_MODE) $(ENABLE_VIDEO_TRACK) $(ENABLE_VIDEO_USES_ELEMENT_FULLSCREEN) $(ENABLE_WEB_API_STATISTICS) $(ENABLE_WEB_AUDIO) $(ENABLE_WEB_AUTHN) $(ENABLE_WEB_RTC) $(ENABLE_WEBASSEMBLY) $(ENABLE_WEBASSEMBLY_STREAMING_API) $(ENABLE_WEBGL) $(ENABLE_WEBGL2) $(ENABLE_WEBGPU) $(ENABLE_WEBMETAL) $(ENABLE_WIRELESS_PLAYBACK_TARGET) $(ENABLE_XSLT);
diff --git a/Source/WebCore/CMakeLists.txt b/Source/WebCore/CMakeLists.txt
index 0de5943..482cf6f 100644
--- a/Source/WebCore/CMakeLists.txt
+++ b/Source/WebCore/CMakeLists.txt
@@ -58,6 +58,7 @@
     "${WEBCORE_DIR}/Modules/websockets"
     "${WEBCORE_DIR}/Modules/webvr"
     "${WEBCORE_DIR}/accessibility"
+    "${WEBCORE_DIR}/accessibility/isolatedtree"
     "${WEBCORE_DIR}/animation"
     "${WEBCORE_DIR}/bindings"
     "${WEBCORE_DIR}/bindings/js"
diff --git a/Source/WebCore/ChangeLog b/Source/WebCore/ChangeLog
index aaa1139..8a65aa4 100644
--- a/Source/WebCore/ChangeLog
+++ b/Source/WebCore/ChangeLog
@@ -1,3 +1,78 @@
+2019-01-27  Chris Fleizach  <cfleizach@apple.com>
+
+        AX: Introduce a static accessibility tree
+        https://bugs.webkit.org/show_bug.cgi?id=193348
+        <rdar://problem/47203295>
+
+        Reviewed by Ryosuke Niwa.
+
+        In order to improve performance when requesting the accessibility hierarchy, we introduce the idea of a "static accessibility tree" which 
+        could be accessed on a different thread by assistive technologies.
+        That is accomplished by storing all the data needed to answer accessibility attribute queries in a static object that mirrors the 
+        "live" AccessibilityObjects (which interact with both DOM and Render trees).
+        These static objects are generally created after layout is done and final tasks are being performed. They are then stored in the static tree 
+        representation and able to be read from anywhere.
+        Tactically this is done with AXIsolatedTreeNodes inside of an AXIsolatedTree. The TreeNodes implement an AccessibilityObjectInterface shared 
+        with AccessibilityObject.
+        This allows the wrappers to access either one depending on conditions and platforms without significant code duplication or re-organization.
+
+        * CMakeLists.txt:
+        * Configurations/FeatureDefines.xcconfig:
+        * Sources.txt:
+        * WebCore.xcodeproj/project.pbxproj:
+        * accessibility/AXObjectCache.cpp:
+        (WebCore::AXObjectCache::remove):
+        (WebCore::AXObjectCache::createIsolatedAccessibilityTree):
+        (WebCore::AXObjectCache::generateStaticAccessibilityTreeIfNeeded):
+        * accessibility/AXObjectCache.h:
+        * accessibility/AccessibilityObject.h:
+        * accessibility/AccessibilityObjectInterface.h: Added.
+        * accessibility/isolatedtree: Added.
+        * accessibility/isolatedtree/AXIsolatedTree.cpp: Added.
+        (WebCore::AXIsolatedTree::treeCache):
+        (WebCore::AXIsolatedTree::AXIsolatedTree):
+        (WebCore::AXIsolatedTree::create):
+        (WebCore::AXIsolatedTree::treeForID):
+        (WebCore::AXIsolatedTree::treeForPageID):
+        (WebCore::AXIsolatedTree::nodeForID const):
+        (WebCore::AXIsolatedTree::rootNode):
+        (WebCore::AXIsolatedTree::removeNode):
+        (WebCore::AXIsolatedTree::appendNodeChanges):
+        (WebCore::AXIsolatedTree::applyPendingChanges):
+        * accessibility/isolatedtree/AXIsolatedTree.h: Added.
+        (WebCore::AXIsolatedTree::treeIdentifier const):
+        * accessibility/isolatedtree/AXIsolatedTreeNode.cpp: Added.
+            To note: we don't mark the attribute map const because even though attributes don't change after initial creation,
+            we may copy an existing node and replace specific values.
+        (WebCore::AXIsolatedTreeNode::AXIsolatedTreeNode):
+        (WebCore::AXIsolatedTreeNode::create):
+        (WebCore::AXIsolatedTreeNode::initializeAttributeData):
+        (WebCore::AXIsolatedTreeNode::setProperty):
+        (WebCore::AXIsolatedTreeNode::doubleAttributeValue const):
+        (WebCore::AXIsolatedTreeNode::unsignedAttributeValue const):
+        (WebCore::AXIsolatedTreeNode::boolAttributeValue const):
+        (WebCore::AXIsolatedTreeNode::stringAttributeValue const):
+        (WebCore::AXIsolatedTreeNode::intAttributeValue const):
+        * accessibility/isolatedtree/AXIsolatedTreeNode.h: Added.
+        * accessibility/mac/AXObjectCacheMac.mm:
+        (WebCore::AXObjectCache::associateIsolatedTreeNode):
+        * accessibility/mac/WebAccessibilityObjectWrapperBase.h:
+        * accessibility/mac/WebAccessibilityObjectWrapperBase.mm:
+        (-[WebAccessibilityObjectWrapperBase initWithAccessibilityObject:]):
+        (-[WebAccessibilityObjectWrapperBase isolatedTreeNode]):
+        (-[WebAccessibilityObjectWrapperBase detach]):
+        (-[WebAccessibilityObjectWrapperBase updateObjectBackingStore]):
+        (-[WebAccessibilityObjectWrapperBase axBackingObject]):
+        (-[WebAccessibilityObjectWrapperBase baseAccessibilityDescription]):
+        * accessibility/mac/WebAccessibilityObjectWrapperMac.mm:
+        (-[WebAccessibilityObjectWrapper role]):
+        (-[WebAccessibilityObjectWrapper subrole]):
+        (-[WebAccessibilityObjectWrapper roleDescription]):
+        (-[WebAccessibilityObjectWrapper accessibilityAttributeValue:]):
+        * dom/Document.cpp:
+        (WebCore::Document::pageID const):
+        * dom/Document.h:
+
 2019-01-26  Simon Fraser  <simon.fraser@apple.com>
 
         Allow scrolling tree nodes to exist in a detached state
diff --git a/Source/WebCore/Configurations/FeatureDefines.xcconfig b/Source/WebCore/Configurations/FeatureDefines.xcconfig
index 9122a25..230623e 100644
--- a/Source/WebCore/Configurations/FeatureDefines.xcconfig
+++ b/Source/WebCore/Configurations/FeatureDefines.xcconfig
@@ -40,6 +40,8 @@
 ENABLE_ACCELERATED_OVERFLOW_SCROLLING = $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING_$(WK_COCOA_TOUCH));
 ENABLE_ACCELERATED_OVERFLOW_SCROLLING_cocoatouch = ENABLE_ACCELERATED_OVERFLOW_SCROLLING;
 
+ENABLE_ACCESSIBILITY_ISOLATED_TREE = ;
+
 ENABLE_APPLE_PAY = $(ENABLE_APPLE_PAY_$(WK_PLATFORM_NAME));
 ENABLE_APPLE_PAY_iphoneos = ENABLE_APPLE_PAY;
 ENABLE_APPLE_PAY_iphonesimulator = ENABLE_APPLE_PAY;
@@ -407,4 +409,4 @@
 
 ENABLE_XSLT = ENABLE_XSLT;
 
-FEATURE_DEFINES = $(ENABLE_3D_TRANSFORMS) $(ENABLE_ACCELERATED_2D_CANVAS) $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING) $(ENABLE_APPLE_PAY) $(ENABLE_APPLE_PAY_SESSION_V3) $(ENABLE_APPLE_PAY_SESSION_V4) $(ENABLE_APPLICATION_MANIFEST) $(ENABLE_ATTACHMENT_ELEMENT) $(ENABLE_AVF_CAPTIONS) $(ENABLE_C_LOOP) $(ENABLE_CACHE_PARTITIONING) $(ENABLE_CHANNEL_MESSAGING) $(ENABLE_CONTENT_FILTERING) $(ENABLE_CSS3_TEXT) $(ENABLE_CSS_BOX_DECORATION_BREAK) $(ENABLE_CSS_COMPOSITING) $(ENABLE_CSS_DEVICE_ADAPTATION) $(ENABLE_CSS_IMAGE_ORIENTATION) $(ENABLE_CSS_IMAGE_RESOLUTION) $(ENABLE_CSS_PAINTING_API) $(ENABLE_CSS_SCROLL_SNAP) $(ENABLE_CSS_SELECTORS_LEVEL4) $(ENABLE_CSS_TRAILING_WORD) $(ENABLE_CSS_TYPED_OM) $(ENABLE_CURSOR_VISIBILITY) $(ENABLE_CUSTOM_SCHEME_HANDLER) $(ENABLE_CSS_CONIC_GRADIENTS) $(ENABLE_DARK_MODE_CSS) $(ENABLE_DASHBOARD_SUPPORT) $(ENABLE_DATA_INTERACTION) $(ENABLE_DATACUE_VALUE) $(ENABLE_DATALIST_ELEMENT) $(ENABLE_DEVICE_ORIENTATION) $(ENABLE_DRAG_SUPPORT) $(ENABLE_ENCRYPTED_MEDIA) $(ENABLE_EXPERIMENTAL_FEATURES) $(ENABLE_FAST_JIT_PERMISSIONS) $(ENABLE_FILTERS_LEVEL_2) $(ENABLE_FTL_JIT) $(ENABLE_FULLSCREEN_API) $(ENABLE_GAMEPAD) $(ENABLE_GEOLOCATION) $(ENABLE_ICONDATABASE) $(ENABLE_INDEXED_DATABASE) $(ENABLE_INDEXED_DATABASE_IN_WORKERS) $(ENABLE_INPUT_TYPE_COLOR) $(ENABLE_INPUT_TYPE_DATE) $(ENABLE_INPUT_TYPE_DATETIME_INCOMPLETE) $(ENABLE_INPUT_TYPE_DATETIMELOCAL) $(ENABLE_INPUT_TYPE_MONTH) $(ENABLE_INPUT_TYPE_TIME) $(ENABLE_INPUT_TYPE_WEEK) $(ENABLE_INTERSECTION_OBSERVER) $(ENABLE_INTL) $(ENABLE_IOS_GESTURE_EVENTS) $(ENABLE_IOS_TOUCH_EVENTS) $(ENABLE_JIT) $(ENABLE_KEYBOARD_CODE_ATTRIBUTE) $(ENABLE_KEYBOARD_KEY_ATTRIBUTE) $(ENABLE_LAYOUT_FORMATTING_CONTEXT) $(ENABLE_LEGACY_CSS_VENDOR_PREFIXES) $(ENABLE_LEGACY_CUSTOM_PROTOCOL_MANAGER) $(ENABLE_LEGACY_ENCRYPTED_MEDIA) $(ENABLE_LETTERPRESS) $(ENABLE_MAC_GESTURE_EVENTS) $(ENABLE_MAC_VIDEO_TOOLBOX) $(ENABLE_MATHML) $(ENABLE_MEDIA_CAPTURE) $(ENABLE_MEDIA_CONTROLS_SCRIPT) $(ENABLE_MEDIA_SESSION) $(ENABLE_MEDIA_SOURCE) $(ENABLE_MEDIA_STATISTICS) $(ENABLE_MEDIA_STREAM) $(ENABLE_METER_ELEMENT) $(ENABLE_MHTML) $(ENABLE_MOUSE_CURSOR_SCALE) $(ENABLE_NAVIGATOR_CONTENT_UTILS) $(ENABLE_NAVIGATOR_STANDALONE) $(ENABLE_NOTIFICATIONS) $(ENABLE_PAYMENT_REQUEST) $(ENABLE_PDFKIT_PLUGIN) $(ENABLE_POINTER_EVENTS) $(ENABLE_POINTER_LOCK) $(ENABLE_PUBLIC_SUFFIX_LIST) $(ENABLE_QUOTA) $(ENABLE_REMOTE_INSPECTOR) $(ENABLE_RESOLUTION_MEDIA_QUERY) $(ENABLE_RESOURCE_LOAD_STATISTICS) $(ENABLE_RESOURCE_USAGE) $(ENABLE_RUBBER_BANDING) $(ENABLE_SERVICE_CONTROLS) $(ENABLE_SERVICE_WORKER) $(ENABLE_SPEECH_SYNTHESIS) $(ENABLE_STREAMS_API) $(ENABLE_WEB_CRYPTO) $(ENABLE_SVG_FONTS) $(ENABLE_TELEPHONE_NUMBER_DETECTION) $(ENABLE_TEXT_AUTOSIZING) $(ENABLE_TOUCH_EVENTS) $(ENABLE_USER_MESSAGE_HANDLERS) $(ENABLE_USERSELECT_ALL) $(ENABLE_VARIATION_FONTS) $(ENABLE_VIDEO) $(ENABLE_VIDEO_PRESENTATION_MODE) $(ENABLE_VIDEO_TRACK) $(ENABLE_VIDEO_USES_ELEMENT_FULLSCREEN) $(ENABLE_WEB_API_STATISTICS) $(ENABLE_WEB_AUDIO) $(ENABLE_WEB_AUTHN) $(ENABLE_WEB_RTC) $(ENABLE_WEBASSEMBLY) $(ENABLE_WEBASSEMBLY_STREAMING_API) $(ENABLE_WEBGL) $(ENABLE_WEBGL2) $(ENABLE_WEBGPU) $(ENABLE_WEBMETAL) $(ENABLE_WIRELESS_PLAYBACK_TARGET) $(ENABLE_XSLT);
+FEATURE_DEFINES = $(ENABLE_3D_TRANSFORMS) $(ENABLE_ACCELERATED_2D_CANVAS) $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING) $(ENABLE_ACCESSIBILITY_ISOLATED_TREE) $(ENABLE_APPLE_PAY) $(ENABLE_APPLE_PAY_SESSION_V3) $(ENABLE_APPLE_PAY_SESSION_V4) $(ENABLE_APPLICATION_MANIFEST) $(ENABLE_ATTACHMENT_ELEMENT) $(ENABLE_AVF_CAPTIONS) $(ENABLE_C_LOOP) $(ENABLE_CACHE_PARTITIONING) $(ENABLE_CHANNEL_MESSAGING) $(ENABLE_CONTENT_FILTERING) $(ENABLE_CSS3_TEXT) $(ENABLE_CSS_BOX_DECORATION_BREAK) $(ENABLE_CSS_COMPOSITING) $(ENABLE_CSS_DEVICE_ADAPTATION) $(ENABLE_CSS_IMAGE_ORIENTATION) $(ENABLE_CSS_IMAGE_RESOLUTION) $(ENABLE_CSS_PAINTING_API) $(ENABLE_CSS_SCROLL_SNAP) $(ENABLE_CSS_SELECTORS_LEVEL4) $(ENABLE_CSS_TRAILING_WORD) $(ENABLE_CSS_TYPED_OM) $(ENABLE_CURSOR_VISIBILITY) $(ENABLE_CUSTOM_SCHEME_HANDLER) $(ENABLE_CSS_CONIC_GRADIENTS) $(ENABLE_DARK_MODE_CSS) $(ENABLE_DASHBOARD_SUPPORT) $(ENABLE_DATA_INTERACTION) $(ENABLE_DATACUE_VALUE) $(ENABLE_DATALIST_ELEMENT) $(ENABLE_DEVICE_ORIENTATION) $(ENABLE_DRAG_SUPPORT) $(ENABLE_ENCRYPTED_MEDIA) $(ENABLE_EXPERIMENTAL_FEATURES) $(ENABLE_FAST_JIT_PERMISSIONS) $(ENABLE_FILTERS_LEVEL_2) $(ENABLE_FTL_JIT) $(ENABLE_FULLSCREEN_API) $(ENABLE_GAMEPAD) $(ENABLE_GEOLOCATION) $(ENABLE_ICONDATABASE) $(ENABLE_INDEXED_DATABASE) $(ENABLE_INDEXED_DATABASE_IN_WORKERS) $(ENABLE_INPUT_TYPE_COLOR) $(ENABLE_INPUT_TYPE_DATE) $(ENABLE_INPUT_TYPE_DATETIME_INCOMPLETE) $(ENABLE_INPUT_TYPE_DATETIMELOCAL) $(ENABLE_INPUT_TYPE_MONTH) $(ENABLE_INPUT_TYPE_TIME) $(ENABLE_INPUT_TYPE_WEEK) $(ENABLE_INTERSECTION_OBSERVER) $(ENABLE_INTL) $(ENABLE_IOS_GESTURE_EVENTS) $(ENABLE_IOS_TOUCH_EVENTS) $(ENABLE_JIT) $(ENABLE_KEYBOARD_CODE_ATTRIBUTE) $(ENABLE_KEYBOARD_KEY_ATTRIBUTE) $(ENABLE_LAYOUT_FORMATTING_CONTEXT) $(ENABLE_LEGACY_CSS_VENDOR_PREFIXES) $(ENABLE_LEGACY_CUSTOM_PROTOCOL_MANAGER) $(ENABLE_LEGACY_ENCRYPTED_MEDIA) $(ENABLE_LETTERPRESS) $(ENABLE_MAC_GESTURE_EVENTS) $(ENABLE_MAC_VIDEO_TOOLBOX) $(ENABLE_MATHML) $(ENABLE_MEDIA_CAPTURE) $(ENABLE_MEDIA_CONTROLS_SCRIPT) $(ENABLE_MEDIA_SESSION) $(ENABLE_MEDIA_SOURCE) $(ENABLE_MEDIA_STATISTICS) $(ENABLE_MEDIA_STREAM) $(ENABLE_METER_ELEMENT) $(ENABLE_MHTML) $(ENABLE_MOUSE_CURSOR_SCALE) $(ENABLE_NAVIGATOR_CONTENT_UTILS) $(ENABLE_NAVIGATOR_STANDALONE) $(ENABLE_NOTIFICATIONS) $(ENABLE_PAYMENT_REQUEST) $(ENABLE_PDFKIT_PLUGIN) $(ENABLE_POINTER_EVENTS) $(ENABLE_POINTER_LOCK) $(ENABLE_PUBLIC_SUFFIX_LIST) $(ENABLE_QUOTA) $(ENABLE_REMOTE_INSPECTOR) $(ENABLE_RESOLUTION_MEDIA_QUERY) $(ENABLE_RESOURCE_LOAD_STATISTICS) $(ENABLE_RESOURCE_USAGE) $(ENABLE_RUBBER_BANDING) $(ENABLE_SERVICE_CONTROLS) $(ENABLE_SERVICE_WORKER) $(ENABLE_SPEECH_SYNTHESIS) $(ENABLE_STREAMS_API) $(ENABLE_WEB_CRYPTO) $(ENABLE_SVG_FONTS) $(ENABLE_TELEPHONE_NUMBER_DETECTION) $(ENABLE_TEXT_AUTOSIZING) $(ENABLE_TOUCH_EVENTS) $(ENABLE_USER_MESSAGE_HANDLERS) $(ENABLE_USERSELECT_ALL) $(ENABLE_VARIATION_FONTS) $(ENABLE_VIDEO) $(ENABLE_VIDEO_PRESENTATION_MODE) $(ENABLE_VIDEO_TRACK) $(ENABLE_VIDEO_USES_ELEMENT_FULLSCREEN) $(ENABLE_WEB_API_STATISTICS) $(ENABLE_WEB_AUDIO) $(ENABLE_WEB_AUTHN) $(ENABLE_WEB_RTC) $(ENABLE_WEBASSEMBLY) $(ENABLE_WEBASSEMBLY_STREAMING_API) $(ENABLE_WEBGL) $(ENABLE_WEBGL2) $(ENABLE_WEBGPU) $(ENABLE_WEBMETAL) $(ENABLE_WIRELESS_PLAYBACK_TARGET) $(ENABLE_XSLT);
diff --git a/Source/WebCore/PAL/ChangeLog b/Source/WebCore/PAL/ChangeLog
index cd4dc6c..83c5c26 100644
--- a/Source/WebCore/PAL/ChangeLog
+++ b/Source/WebCore/PAL/ChangeLog
@@ -1,3 +1,13 @@
+2019-01-27  Chris Fleizach  <cfleizach@apple.com>
+
+        AX: Introduce isolated accessibility tree
+        https://bugs.webkit.org/show_bug.cgi?id=193348
+        <rdar://problem/47203295>
+
+        Reviewed by Ryosuke Niwa.
+
+        * Configurations/FeatureDefines.xcconfig:
+
 2019-01-25  Alex Christensen  <achristensen@webkit.org>
 
         Move logic from ChildProcess::setApplicationIsDaemon to NetworkProcess::initializeProcess
diff --git a/Source/WebCore/PAL/Configurations/FeatureDefines.xcconfig b/Source/WebCore/PAL/Configurations/FeatureDefines.xcconfig
index 9122a25..230623e 100644
--- a/Source/WebCore/PAL/Configurations/FeatureDefines.xcconfig
+++ b/Source/WebCore/PAL/Configurations/FeatureDefines.xcconfig
@@ -40,6 +40,8 @@
 ENABLE_ACCELERATED_OVERFLOW_SCROLLING = $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING_$(WK_COCOA_TOUCH));
 ENABLE_ACCELERATED_OVERFLOW_SCROLLING_cocoatouch = ENABLE_ACCELERATED_OVERFLOW_SCROLLING;
 
+ENABLE_ACCESSIBILITY_ISOLATED_TREE = ;
+
 ENABLE_APPLE_PAY = $(ENABLE_APPLE_PAY_$(WK_PLATFORM_NAME));
 ENABLE_APPLE_PAY_iphoneos = ENABLE_APPLE_PAY;
 ENABLE_APPLE_PAY_iphonesimulator = ENABLE_APPLE_PAY;
@@ -407,4 +409,4 @@
 
 ENABLE_XSLT = ENABLE_XSLT;
 
-FEATURE_DEFINES = $(ENABLE_3D_TRANSFORMS) $(ENABLE_ACCELERATED_2D_CANVAS) $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING) $(ENABLE_APPLE_PAY) $(ENABLE_APPLE_PAY_SESSION_V3) $(ENABLE_APPLE_PAY_SESSION_V4) $(ENABLE_APPLICATION_MANIFEST) $(ENABLE_ATTACHMENT_ELEMENT) $(ENABLE_AVF_CAPTIONS) $(ENABLE_C_LOOP) $(ENABLE_CACHE_PARTITIONING) $(ENABLE_CHANNEL_MESSAGING) $(ENABLE_CONTENT_FILTERING) $(ENABLE_CSS3_TEXT) $(ENABLE_CSS_BOX_DECORATION_BREAK) $(ENABLE_CSS_COMPOSITING) $(ENABLE_CSS_DEVICE_ADAPTATION) $(ENABLE_CSS_IMAGE_ORIENTATION) $(ENABLE_CSS_IMAGE_RESOLUTION) $(ENABLE_CSS_PAINTING_API) $(ENABLE_CSS_SCROLL_SNAP) $(ENABLE_CSS_SELECTORS_LEVEL4) $(ENABLE_CSS_TRAILING_WORD) $(ENABLE_CSS_TYPED_OM) $(ENABLE_CURSOR_VISIBILITY) $(ENABLE_CUSTOM_SCHEME_HANDLER) $(ENABLE_CSS_CONIC_GRADIENTS) $(ENABLE_DARK_MODE_CSS) $(ENABLE_DASHBOARD_SUPPORT) $(ENABLE_DATA_INTERACTION) $(ENABLE_DATACUE_VALUE) $(ENABLE_DATALIST_ELEMENT) $(ENABLE_DEVICE_ORIENTATION) $(ENABLE_DRAG_SUPPORT) $(ENABLE_ENCRYPTED_MEDIA) $(ENABLE_EXPERIMENTAL_FEATURES) $(ENABLE_FAST_JIT_PERMISSIONS) $(ENABLE_FILTERS_LEVEL_2) $(ENABLE_FTL_JIT) $(ENABLE_FULLSCREEN_API) $(ENABLE_GAMEPAD) $(ENABLE_GEOLOCATION) $(ENABLE_ICONDATABASE) $(ENABLE_INDEXED_DATABASE) $(ENABLE_INDEXED_DATABASE_IN_WORKERS) $(ENABLE_INPUT_TYPE_COLOR) $(ENABLE_INPUT_TYPE_DATE) $(ENABLE_INPUT_TYPE_DATETIME_INCOMPLETE) $(ENABLE_INPUT_TYPE_DATETIMELOCAL) $(ENABLE_INPUT_TYPE_MONTH) $(ENABLE_INPUT_TYPE_TIME) $(ENABLE_INPUT_TYPE_WEEK) $(ENABLE_INTERSECTION_OBSERVER) $(ENABLE_INTL) $(ENABLE_IOS_GESTURE_EVENTS) $(ENABLE_IOS_TOUCH_EVENTS) $(ENABLE_JIT) $(ENABLE_KEYBOARD_CODE_ATTRIBUTE) $(ENABLE_KEYBOARD_KEY_ATTRIBUTE) $(ENABLE_LAYOUT_FORMATTING_CONTEXT) $(ENABLE_LEGACY_CSS_VENDOR_PREFIXES) $(ENABLE_LEGACY_CUSTOM_PROTOCOL_MANAGER) $(ENABLE_LEGACY_ENCRYPTED_MEDIA) $(ENABLE_LETTERPRESS) $(ENABLE_MAC_GESTURE_EVENTS) $(ENABLE_MAC_VIDEO_TOOLBOX) $(ENABLE_MATHML) $(ENABLE_MEDIA_CAPTURE) $(ENABLE_MEDIA_CONTROLS_SCRIPT) $(ENABLE_MEDIA_SESSION) $(ENABLE_MEDIA_SOURCE) $(ENABLE_MEDIA_STATISTICS) $(ENABLE_MEDIA_STREAM) $(ENABLE_METER_ELEMENT) $(ENABLE_MHTML) $(ENABLE_MOUSE_CURSOR_SCALE) $(ENABLE_NAVIGATOR_CONTENT_UTILS) $(ENABLE_NAVIGATOR_STANDALONE) $(ENABLE_NOTIFICATIONS) $(ENABLE_PAYMENT_REQUEST) $(ENABLE_PDFKIT_PLUGIN) $(ENABLE_POINTER_EVENTS) $(ENABLE_POINTER_LOCK) $(ENABLE_PUBLIC_SUFFIX_LIST) $(ENABLE_QUOTA) $(ENABLE_REMOTE_INSPECTOR) $(ENABLE_RESOLUTION_MEDIA_QUERY) $(ENABLE_RESOURCE_LOAD_STATISTICS) $(ENABLE_RESOURCE_USAGE) $(ENABLE_RUBBER_BANDING) $(ENABLE_SERVICE_CONTROLS) $(ENABLE_SERVICE_WORKER) $(ENABLE_SPEECH_SYNTHESIS) $(ENABLE_STREAMS_API) $(ENABLE_WEB_CRYPTO) $(ENABLE_SVG_FONTS) $(ENABLE_TELEPHONE_NUMBER_DETECTION) $(ENABLE_TEXT_AUTOSIZING) $(ENABLE_TOUCH_EVENTS) $(ENABLE_USER_MESSAGE_HANDLERS) $(ENABLE_USERSELECT_ALL) $(ENABLE_VARIATION_FONTS) $(ENABLE_VIDEO) $(ENABLE_VIDEO_PRESENTATION_MODE) $(ENABLE_VIDEO_TRACK) $(ENABLE_VIDEO_USES_ELEMENT_FULLSCREEN) $(ENABLE_WEB_API_STATISTICS) $(ENABLE_WEB_AUDIO) $(ENABLE_WEB_AUTHN) $(ENABLE_WEB_RTC) $(ENABLE_WEBASSEMBLY) $(ENABLE_WEBASSEMBLY_STREAMING_API) $(ENABLE_WEBGL) $(ENABLE_WEBGL2) $(ENABLE_WEBGPU) $(ENABLE_WEBMETAL) $(ENABLE_WIRELESS_PLAYBACK_TARGET) $(ENABLE_XSLT);
+FEATURE_DEFINES = $(ENABLE_3D_TRANSFORMS) $(ENABLE_ACCELERATED_2D_CANVAS) $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING) $(ENABLE_ACCESSIBILITY_ISOLATED_TREE) $(ENABLE_APPLE_PAY) $(ENABLE_APPLE_PAY_SESSION_V3) $(ENABLE_APPLE_PAY_SESSION_V4) $(ENABLE_APPLICATION_MANIFEST) $(ENABLE_ATTACHMENT_ELEMENT) $(ENABLE_AVF_CAPTIONS) $(ENABLE_C_LOOP) $(ENABLE_CACHE_PARTITIONING) $(ENABLE_CHANNEL_MESSAGING) $(ENABLE_CONTENT_FILTERING) $(ENABLE_CSS3_TEXT) $(ENABLE_CSS_BOX_DECORATION_BREAK) $(ENABLE_CSS_COMPOSITING) $(ENABLE_CSS_DEVICE_ADAPTATION) $(ENABLE_CSS_IMAGE_ORIENTATION) $(ENABLE_CSS_IMAGE_RESOLUTION) $(ENABLE_CSS_PAINTING_API) $(ENABLE_CSS_SCROLL_SNAP) $(ENABLE_CSS_SELECTORS_LEVEL4) $(ENABLE_CSS_TRAILING_WORD) $(ENABLE_CSS_TYPED_OM) $(ENABLE_CURSOR_VISIBILITY) $(ENABLE_CUSTOM_SCHEME_HANDLER) $(ENABLE_CSS_CONIC_GRADIENTS) $(ENABLE_DARK_MODE_CSS) $(ENABLE_DASHBOARD_SUPPORT) $(ENABLE_DATA_INTERACTION) $(ENABLE_DATACUE_VALUE) $(ENABLE_DATALIST_ELEMENT) $(ENABLE_DEVICE_ORIENTATION) $(ENABLE_DRAG_SUPPORT) $(ENABLE_ENCRYPTED_MEDIA) $(ENABLE_EXPERIMENTAL_FEATURES) $(ENABLE_FAST_JIT_PERMISSIONS) $(ENABLE_FILTERS_LEVEL_2) $(ENABLE_FTL_JIT) $(ENABLE_FULLSCREEN_API) $(ENABLE_GAMEPAD) $(ENABLE_GEOLOCATION) $(ENABLE_ICONDATABASE) $(ENABLE_INDEXED_DATABASE) $(ENABLE_INDEXED_DATABASE_IN_WORKERS) $(ENABLE_INPUT_TYPE_COLOR) $(ENABLE_INPUT_TYPE_DATE) $(ENABLE_INPUT_TYPE_DATETIME_INCOMPLETE) $(ENABLE_INPUT_TYPE_DATETIMELOCAL) $(ENABLE_INPUT_TYPE_MONTH) $(ENABLE_INPUT_TYPE_TIME) $(ENABLE_INPUT_TYPE_WEEK) $(ENABLE_INTERSECTION_OBSERVER) $(ENABLE_INTL) $(ENABLE_IOS_GESTURE_EVENTS) $(ENABLE_IOS_TOUCH_EVENTS) $(ENABLE_JIT) $(ENABLE_KEYBOARD_CODE_ATTRIBUTE) $(ENABLE_KEYBOARD_KEY_ATTRIBUTE) $(ENABLE_LAYOUT_FORMATTING_CONTEXT) $(ENABLE_LEGACY_CSS_VENDOR_PREFIXES) $(ENABLE_LEGACY_CUSTOM_PROTOCOL_MANAGER) $(ENABLE_LEGACY_ENCRYPTED_MEDIA) $(ENABLE_LETTERPRESS) $(ENABLE_MAC_GESTURE_EVENTS) $(ENABLE_MAC_VIDEO_TOOLBOX) $(ENABLE_MATHML) $(ENABLE_MEDIA_CAPTURE) $(ENABLE_MEDIA_CONTROLS_SCRIPT) $(ENABLE_MEDIA_SESSION) $(ENABLE_MEDIA_SOURCE) $(ENABLE_MEDIA_STATISTICS) $(ENABLE_MEDIA_STREAM) $(ENABLE_METER_ELEMENT) $(ENABLE_MHTML) $(ENABLE_MOUSE_CURSOR_SCALE) $(ENABLE_NAVIGATOR_CONTENT_UTILS) $(ENABLE_NAVIGATOR_STANDALONE) $(ENABLE_NOTIFICATIONS) $(ENABLE_PAYMENT_REQUEST) $(ENABLE_PDFKIT_PLUGIN) $(ENABLE_POINTER_EVENTS) $(ENABLE_POINTER_LOCK) $(ENABLE_PUBLIC_SUFFIX_LIST) $(ENABLE_QUOTA) $(ENABLE_REMOTE_INSPECTOR) $(ENABLE_RESOLUTION_MEDIA_QUERY) $(ENABLE_RESOURCE_LOAD_STATISTICS) $(ENABLE_RESOURCE_USAGE) $(ENABLE_RUBBER_BANDING) $(ENABLE_SERVICE_CONTROLS) $(ENABLE_SERVICE_WORKER) $(ENABLE_SPEECH_SYNTHESIS) $(ENABLE_STREAMS_API) $(ENABLE_WEB_CRYPTO) $(ENABLE_SVG_FONTS) $(ENABLE_TELEPHONE_NUMBER_DETECTION) $(ENABLE_TEXT_AUTOSIZING) $(ENABLE_TOUCH_EVENTS) $(ENABLE_USER_MESSAGE_HANDLERS) $(ENABLE_USERSELECT_ALL) $(ENABLE_VARIATION_FONTS) $(ENABLE_VIDEO) $(ENABLE_VIDEO_PRESENTATION_MODE) $(ENABLE_VIDEO_TRACK) $(ENABLE_VIDEO_USES_ELEMENT_FULLSCREEN) $(ENABLE_WEB_API_STATISTICS) $(ENABLE_WEB_AUDIO) $(ENABLE_WEB_AUTHN) $(ENABLE_WEB_RTC) $(ENABLE_WEBASSEMBLY) $(ENABLE_WEBASSEMBLY_STREAMING_API) $(ENABLE_WEBGL) $(ENABLE_WEBGL2) $(ENABLE_WEBGPU) $(ENABLE_WEBMETAL) $(ENABLE_WIRELESS_PLAYBACK_TARGET) $(ENABLE_XSLT);
diff --git a/Source/WebCore/Sources.txt b/Source/WebCore/Sources.txt
index e49d19c..de5421f 100644
--- a/Source/WebCore/Sources.txt
+++ b/Source/WebCore/Sources.txt
@@ -405,6 +405,8 @@
 accessibility/AccessibilityTree.cpp
 accessibility/AccessibilityTreeItem.cpp
 accessibility/AccessibleSetValueEvent.cpp
+accessibility/isolatedtree/AXIsolatedTree.cpp
+accessibility/isolatedtree/AXIsolatedTreeNode.cpp
 
 animation/AnimationEffect.cpp
 animation/AnimationPlaybackEvent.cpp
diff --git a/Source/WebCore/WebCore.xcodeproj/project.pbxproj b/Source/WebCore/WebCore.xcodeproj/project.pbxproj
index 990dcda..7ed125d 100644
--- a/Source/WebCore/WebCore.xcodeproj/project.pbxproj
+++ b/Source/WebCore/WebCore.xcodeproj/project.pbxproj
@@ -707,6 +707,7 @@
 		26F9A83818A046AC00AEB88A /* ViewportConfiguration.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 26F9A83618A046AC00AEB88A /* ViewportConfiguration.cpp */; };
 		26F9A83918A046AC00AEB88A /* ViewportConfiguration.h in Headers */ = {isa = PBXBuildFile; fileRef = 26F9A83718A046AC00AEB88A /* ViewportConfiguration.h */; settings = {ATTRIBUTES = (Private, ); }; };
 		2914E3081CAB5A440049966F /* AccessibilityAttachment.h in Headers */ = {isa = PBXBuildFile; fileRef = 2914E3061CAB5A440049966F /* AccessibilityAttachment.h */; };
+		2936BF5C21D69E4B004A8FC9 /* AccessibilityObjectInterface.h in Headers */ = {isa = PBXBuildFile; fileRef = 2936BF5A21D6999E004A8FC9 /* AccessibilityObjectInterface.h */; settings = {ATTRIBUTES = (Private, ); }; };
 		293EAE1F1356B2FE0067ACF9 /* RuntimeApplicationChecks.h in Headers */ = {isa = PBXBuildFile; fileRef = 293EAE1E1356B2FE0067ACF9 /* RuntimeApplicationChecks.h */; settings = {ATTRIBUTES = (Private, ); }; };
 		29489FC712C00F0300D83F0F /* AccessibilityScrollView.h in Headers */ = {isa = PBXBuildFile; fileRef = 29489FC512C00F0300D83F0F /* AccessibilityScrollView.h */; };
 		297BE3D516C03C08003316BD /* PlatformSpeechSynthesisUtterance.h in Headers */ = {isa = PBXBuildFile; fileRef = 2527CC9116BF8BA1009DDAC0 /* PlatformSpeechSynthesisUtterance.h */; };
@@ -728,6 +729,8 @@
 		29A812430FBB9C1D00510293 /* AccessibilityListBox.h in Headers */ = {isa = PBXBuildFile; fileRef = 29A812250FBB9C1D00510293 /* AccessibilityListBox.h */; };
 		29A812490FBB9CA900510293 /* WebAccessibilityObjectWrapperBase.h in Headers */ = {isa = PBXBuildFile; fileRef = 29A812450FBB9CA900510293 /* WebAccessibilityObjectWrapperBase.h */; settings = {ATTRIBUTES = (Private, ); }; };
 		29ACB212143E7128006BCA5F /* AccessibilityMockObject.h in Headers */ = {isa = PBXBuildFile; fileRef = 29ACB211143E7128006BCA5F /* AccessibilityMockObject.h */; };
+		29AE212D21AB9EEB00869283 /* AXIsolatedTree.h in Headers */ = {isa = PBXBuildFile; fileRef = 29AE212B21AB9EEB00869283 /* AXIsolatedTree.h */; settings = {ATTRIBUTES = (Private, ); }; };
+		29AE213521ABA48A00869283 /* AXIsolatedTreeNode.h in Headers */ = {isa = PBXBuildFile; fileRef = 29AE213321ABA48A00869283 /* AXIsolatedTreeNode.h */; settings = {ATTRIBUTES = (Private, ); }; };
 		29D7BCFA1444AF7D0070619C /* AccessibilitySpinButton.h in Headers */ = {isa = PBXBuildFile; fileRef = 29D7BCF91444AF7D0070619C /* AccessibilitySpinButton.h */; };
 		29FAF4B6195AB08900A522DC /* TextUndoInsertionMarkupMac.h in Headers */ = {isa = PBXBuildFile; fileRef = 29FAF4B5195AB08900A522DC /* TextUndoInsertionMarkupMac.h */; settings = {ATTRIBUTES = (Private, ); }; };
 		2B365C841525119E0091D27B /* RenderSVGEllipse.h in Headers */ = {isa = PBXBuildFile; fileRef = 2B4235A015250F6000DBBCD8 /* RenderSVGEllipse.h */; };
@@ -6595,6 +6598,7 @@
 		28DB9BC72020EA0400E54A07 /* PerformanceServerTiming.idl */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text; path = PerformanceServerTiming.idl; sourceTree = "<group>"; };
 		2914E3051CAB5A440049966F /* AccessibilityAttachment.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = AccessibilityAttachment.cpp; sourceTree = "<group>"; };
 		2914E3061CAB5A440049966F /* AccessibilityAttachment.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = AccessibilityAttachment.h; sourceTree = "<group>"; };
+		2936BF5A21D6999E004A8FC9 /* AccessibilityObjectInterface.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = AccessibilityObjectInterface.h; sourceTree = "<group>"; };
 		293EAE1E1356B2FE0067ACF9 /* RuntimeApplicationChecks.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = RuntimeApplicationChecks.h; sourceTree = "<group>"; };
 		29489FC512C00F0300D83F0F /* AccessibilityScrollView.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = AccessibilityScrollView.h; sourceTree = "<group>"; };
 		29498681195341940072D2BD /* TextUndoInsertionMarkupMac.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = TextUndoInsertionMarkupMac.mm; sourceTree = "<group>"; };
@@ -6642,6 +6646,10 @@
 		29A812470FBB9CA900510293 /* AXObjectCacheMac.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = AXObjectCacheMac.mm; sourceTree = "<group>"; };
 		29ACB211143E7128006BCA5F /* AccessibilityMockObject.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = AccessibilityMockObject.h; sourceTree = "<group>"; };
 		29ACB213143E7498006BCA5F /* AccessibilityMockObject.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = AccessibilityMockObject.cpp; sourceTree = "<group>"; };
+		29AE212B21AB9EEB00869283 /* AXIsolatedTree.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = AXIsolatedTree.h; sourceTree = "<group>"; };
+		29AE212C21AB9EEB00869283 /* AXIsolatedTree.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = AXIsolatedTree.cpp; sourceTree = "<group>"; };
+		29AE213321ABA48A00869283 /* AXIsolatedTreeNode.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = AXIsolatedTreeNode.h; sourceTree = "<group>"; };
+		29AE213421ABA48A00869283 /* AXIsolatedTreeNode.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = AXIsolatedTreeNode.cpp; sourceTree = "<group>"; };
 		29D7BCF51444AF580070619C /* AccessibilitySpinButton.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = AccessibilitySpinButton.cpp; sourceTree = "<group>"; };
 		29D7BCF91444AF7D0070619C /* AccessibilitySpinButton.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = AccessibilitySpinButton.h; sourceTree = "<group>"; };
 		29E04A27BED2F81F98E9022B /* JSBeforeUnloadEvent.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = JSBeforeUnloadEvent.h; sourceTree = "<group>"; };
@@ -17265,6 +17273,7 @@
 		29A812040FBB9B4100510293 /* accessibility */ = {
 			isa = PBXGroup;
 			children = (
+				29AE212921AB9ECB00869283 /* isolatedtree */,
 				AAA728EF16D1D8BC00D3BBC6 /* ios */,
 				29A812050FBB9B5200510293 /* mac */,
 				2981CA9D131822EC00D12F2A /* AccessibilityARIAGrid.cpp */,
@@ -17303,6 +17312,7 @@
 				2985709315CC532D006355CD /* AccessibilityNodeObject.h */,
 				2981CAA5131822EC00D12F2A /* AccessibilityObject.cpp */,
 				29A812180FBB9C1D00510293 /* AccessibilityObject.h */,
+				2936BF5A21D6999E004A8FC9 /* AccessibilityObjectInterface.h */,
 				A409C982116D0DDD007197BD /* AccessibilityProgressIndicator.cpp */,
 				A409C983116D0DDD007197BD /* AccessibilityProgressIndicator.h */,
 				2981CAA6131822EC00D12F2A /* AccessibilityRenderObject.cpp */,
@@ -17358,6 +17368,17 @@
 			path = mac;
 			sourceTree = "<group>";
 		};
+		29AE212921AB9ECB00869283 /* isolatedtree */ = {
+			isa = PBXGroup;
+			children = (
+				29AE212B21AB9EEB00869283 /* AXIsolatedTree.h */,
+				29AE212C21AB9EEB00869283 /* AXIsolatedTree.cpp */,
+				29AE213321ABA48A00869283 /* AXIsolatedTreeNode.h */,
+				29AE213421ABA48A00869283 /* AXIsolatedTreeNode.cpp */,
+			);
+			path = isolatedtree;
+			sourceTree = "<group>";
+		};
 		2D0621461DA639EC00A7FB26 /* legacy */ = {
 			isa = PBXGroup;
 			children = (
@@ -29983,6 +30004,7 @@
 				1AE2AA980A1CDD2D00B42B25 /* JSHTMLImageElement.h in Headers */,
 				A80E7E970A1A83E3007FB8C5 /* JSHTMLInputElement.h in Headers */,
 				836B09561F5F34D9003C3702 /* JSHTMLInputElementEntriesAPI.h in Headers */,
+				2936BF5C21D69E4B004A8FC9 /* AccessibilityObjectInterface.h in Headers */,
 				A6148A7912E41E3B0044A784 /* JSHTMLKeygenElement.h in Headers */,
 				1AE2AB220A1CE63B00B42B25 /* JSHTMLLabelElement.h in Headers */,
 				1AE2AB240A1CE63B00B42B25 /* JSHTMLLegendElement.h in Headers */,
@@ -31438,6 +31460,7 @@
 				51405C89190B014400754F94 /* SelectionRectGatherer.h in Headers */,
 				E44B4BB4141650D7002B1D8B /* SelectorChecker.h in Headers */,
 				432D3FE818A8658400D7DC03 /* SelectorCheckerTestFunctions.h in Headers */,
+				29AE213521ABA48A00869283 /* AXIsolatedTreeNode.h in Headers */,
 				26B999971804D54200D01121 /* SelectorCompiler.h in Headers */,
 				415071581685067300C3C7B3 /* SelectorFilter.h in Headers */,
 				43107BE218CC19DE00CC18E8 /* SelectorPseudoTypeMap.h in Headers */,
@@ -32373,6 +32396,7 @@
 				977E2E0F12F0FC9C00C13380 /* XSSAuditorDelegate.h in Headers */,
 				A5416FE618810EF80009FC5F /* YouTubeEmbedShadowElement.h in Headers */,
 				7A5515F5191830A3009687D2 /* YouTubePluginReplacement.h in Headers */,
+				29AE212D21AB9EEB00869283 /* AXIsolatedTree.h in Headers */,
 				FD537353137B651800008DCE /* ZeroPole.h in Headers */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
diff --git a/Source/WebCore/accessibility/AXObjectCache.cpp b/Source/WebCore/accessibility/AXObjectCache.cpp
index 8090411..f032148 100644
--- a/Source/WebCore/accessibility/AXObjectCache.cpp
+++ b/Source/WebCore/accessibility/AXObjectCache.cpp
@@ -32,6 +32,8 @@
 
 #include "AXObjectCache.h"
 
+#include "AXIsolatedTree.h"
+#include "AXIsolatedTreeNode.h"
 #include "AccessibilityARIAGrid.h"
 #include "AccessibilityARIAGridCell.h"
 #include "AccessibilityARIAGridRow.h"
@@ -115,6 +117,8 @@
 
 using namespace HTMLNames;
 
+const AXID InvalidAXID = 0;
+
 // Post value change notifications for password fields or elements contained in password fields at a 40hz interval to thwart analysis of typing cadence
 static const Seconds accessibilityPasswordValueChangeNotificationInterval { 25_ms };
 static const Seconds accessibilityLiveRegionChangedNotificationInterval { 20_ms };
@@ -723,6 +727,10 @@
     object->setAXObjectID(0);
 
     m_idsInUse.remove(axID);
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+    if (auto pageID = m_document.pageID())
+        AXIsolatedTree::treeForPageID(*pageID)->removeNode(axID);
+#endif
 
     ASSERT(m_objects.size() >= m_idsInUse.size());
 }
@@ -2907,6 +2915,40 @@
     m_deferredFocusedNodeChange.clear();
 }
     
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+Ref<AXIsolatedTreeNode> AXObjectCache::createIsolatedAccessibilityTreeHierarchy(AccessibilityObject& object, AXID parentID, AXIsolatedTree& tree, Vector<Ref<AXIsolatedTreeNode>>& nodeChanges)
+{
+    auto isolatedTreeNode = AXIsolatedTreeNode::create(object);
+    nodeChanges.append(isolatedTreeNode.copyRef());
+
+    isolatedTreeNode->setParent(parentID);
+    associateIsolatedTreeNode(object, isolatedTreeNode, tree.treeIdentifier());
+
+    for (auto child : object.children()) {
+        auto staticChild = createIsolatedAccessibilityTreeHierarchy(*child, isolatedTreeNode->identifier(), tree, nodeChanges);
+        isolatedTreeNode->appendChild(staticChild->identifier());
+    }
+
+    return isolatedTreeNode;
+}
+    
+Ref<AXIsolatedTree> AXObjectCache::generateIsolatedAccessibilityTree()
+{
+    RELEASE_ASSERT(isMainThread());
+
+    auto tree = AXIsolatedTree::treeForPageID(*m_document.pageID());
+    if (!tree)
+        tree = AXIsolatedTree::createTreeForPageID(*m_document.pageID());
+    
+    Vector<Ref<AXIsolatedTreeNode>> nodeChanges;
+    auto root = createIsolatedAccessibilityTreeHierarchy(*rootObject(), InvalidAXID, *tree, nodeChanges);
+    root->setIsRootNode(true);
+    tree->appendNodeChanges(nodeChanges);
+
+    return makeRef(*tree);
+}
+#endif
+    
 void AXObjectCache::deferRecomputeIsIgnoredIfNeeded(Element* element)
 {
     if (!nodeAndRendererAreValid(element))
diff --git a/Source/WebCore/accessibility/AXObjectCache.h b/Source/WebCore/accessibility/AXObjectCache.h
index da4eba8..cf0a183 100644
--- a/Source/WebCore/accessibility/AXObjectCache.h
+++ b/Source/WebCore/accessibility/AXObjectCache.h
@@ -25,6 +25,9 @@
 
 #pragma once
 
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+#include "AXIsolatedTree.h"
+#endif
 #include "AXTextStateChangeIntent.h"
 #include "AccessibilityObject.h"
 #include "Range.h"
@@ -39,6 +42,9 @@
 
 namespace WebCore {
 
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+class AXIsolatedTreeNode;
+#endif
 class Document;
 class HTMLAreaElement;
 class HTMLTextFormControlElement;
@@ -180,6 +186,13 @@
     void deferAttributeChangeIfNeeded(const QualifiedName&, Element*);
     void recomputeIsIgnored(RenderObject* renderer);
 
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+    WEBCORE_EXPORT Ref<AXIsolatedTree> generateIsolatedAccessibilityTree();
+
+    void associateIsolatedTreeNode(AccessibilityObject&, AXIsolatedTreeNode&, AXIsolatedTreeID);
+    Ref<AXIsolatedTreeNode> createIsolatedAccessibilityTreeHierarchy(AccessibilityObject&, AXID, AXIsolatedTree&, Vector<Ref<AXIsolatedTreeNode>>&);
+#endif
+    
 #if HAVE(ACCESSIBILITY)
     WEBCORE_EXPORT static void enableAccessibility();
     WEBCORE_EXPORT static void disableAccessibility();
@@ -187,6 +200,7 @@
     // Enhanced user interface accessibility can be toggled by the assistive technology.
     WEBCORE_EXPORT static void setEnhancedUserInterfaceAccessibility(bool flag);
     
+    // Note: these may be called from a non-main thread concurrently as other readers.
     static bool accessibilityEnabled() { return gAccessibilityEnabled; }
     static bool accessibilityEnhancedUserInterfaceEnabled() { return gAccessibilityEnhancedUserInterfaceEnabled; }
 #else
diff --git a/Source/WebCore/accessibility/AccessibilityObject.h b/Source/WebCore/accessibility/AccessibilityObject.h
index 5e88796..f7fb413 100644
--- a/Source/WebCore/accessibility/AccessibilityObject.h
+++ b/Source/WebCore/accessibility/AccessibilityObject.h
@@ -29,6 +29,7 @@
 
 #pragma once
 
+#include "AccessibilityObjectInterface.h"
 #include "FloatQuad.h"
 #include "HTMLTextFormControlElement.h"
 #include "LayoutRect.h"
@@ -89,157 +90,6 @@
 class ScrollView;
 class Widget;
 
-typedef unsigned AXID;
-
-enum class AccessibilityRole {
-    Annotation = 1,
-    Application,
-    ApplicationAlert,
-    ApplicationAlertDialog,
-    ApplicationDialog,
-    ApplicationGroup,
-    ApplicationLog,
-    ApplicationMarquee,
-    ApplicationStatus,
-    ApplicationTextGroup,
-    ApplicationTimer,
-    Audio,
-    Blockquote,
-    Browser,
-    BusyIndicator,
-    Button,
-    Canvas,
-    Caption,
-    Cell,
-    CheckBox,
-    ColorWell,
-    Column,
-    ColumnHeader,
-    ComboBox,
-    Definition,
-    DescriptionList,
-    DescriptionListTerm,
-    DescriptionListDetail,
-    Details,
-    Directory,
-    DisclosureTriangle,
-    Div,
-    Document,
-    DocumentArticle,
-    DocumentMath,
-    DocumentNote,
-    Drawer,
-    EditableText,
-    Feed,
-    Figure,
-    Footer,
-    Footnote,
-    Form,
-    GraphicsDocument,
-    GraphicsObject,
-    GraphicsSymbol,
-    Grid,
-    GridCell,
-    Group,
-    GrowArea,
-    Heading,
-    HelpTag,
-    HorizontalRule,
-    Ignored,
-    Inline,
-    Image,
-    ImageMap,
-    ImageMapLink,
-    Incrementor,
-    Label,
-    LandmarkBanner,
-    LandmarkComplementary,
-    LandmarkContentInfo,
-    LandmarkDocRegion,
-    LandmarkMain,
-    LandmarkNavigation,
-    LandmarkRegion,
-    LandmarkSearch,
-    Legend,
-    Link,
-    List,
-    ListBox,
-    ListBoxOption,
-    ListItem,
-    ListMarker,
-    Mark,
-    MathElement,
-    Matte,
-    Menu,
-    MenuBar,
-    MenuButton,
-    MenuItem,
-    MenuItemCheckbox,
-    MenuItemRadio,
-    MenuListPopup,
-    MenuListOption,
-    Outline,
-    Paragraph,
-    PopUpButton,
-    Pre,
-    Presentational,
-    ProgressIndicator,
-    RadioButton,
-    RadioGroup,
-    RowHeader,
-    Row,
-    RowGroup,
-    RubyBase,
-    RubyBlock,
-    RubyInline,
-    RubyRun,
-    RubyText,
-    Ruler,
-    RulerMarker,
-    ScrollArea,
-    ScrollBar,
-    SearchField,
-    Sheet,
-    Slider,
-    SliderThumb,
-    SpinButton,
-    SpinButtonPart,
-    SplitGroup,
-    Splitter,
-    StaticText,
-    Summary,
-    Switch,
-    SystemWide,
-    SVGRoot,
-    SVGText,
-    SVGTSpan,
-    SVGTextPath,
-    TabGroup,
-    TabList,
-    TabPanel,
-    Tab,
-    Table,
-    TableHeaderContainer,
-    TextArea,
-    TextGroup,
-    Term,
-    Time,
-    Tree,
-    TreeGrid,
-    TreeItem,
-    TextField,
-    ToggleButton,
-    Toolbar,
-    Unknown,
-    UserInterfaceTooltip,
-    ValueIndicator,
-    Video,
-    WebApplication,
-    WebArea,
-    WebCoreLink,
-    Window,
-};
-
 enum class AccessibilityTextSource {
     Alternative,
     Children,
@@ -491,7 +341,7 @@
     
 bool nodeHasPresentationRole(Node*);
     
-class AccessibilityObject : public RefCounted<AccessibilityObject> {
+class AccessibilityObject : public RefCounted<AccessibilityObject>, public AccessibilityObjectInterface {
 protected:
     AccessibilityObject() = default;
     
@@ -1159,7 +1009,10 @@
     AccessibilityIsIgnoredFromParentData m_isIgnoredFromParentData { };
     bool m_childrenDirty { false };
     bool m_subtreeDirty { false };
-
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+    bool m_isolatedTreeNodeInitialized { false };
+#endif
+    
     void setIsIgnoredFromParentData(AccessibilityIsIgnoredFromParentData& data) { m_isIgnoredFromParentData = data; }
 
     virtual bool computeAccessibilityIsIgnored() const { return true; }
diff --git a/Source/WebCore/accessibility/AccessibilityObjectInterface.h b/Source/WebCore/accessibility/AccessibilityObjectInterface.h
new file mode 100644
index 0000000..7abd287
--- /dev/null
+++ b/Source/WebCore/accessibility/AccessibilityObjectInterface.h
@@ -0,0 +1,191 @@
+/*
+ * Copyright (C) 2019 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ * THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+namespace WebCore {
+
+typedef unsigned AXID;
+extern const AXID InvalidAXID;
+
+enum class AccessibilityRole {
+    Annotation = 1,
+    Application,
+    ApplicationAlert,
+    ApplicationAlertDialog,
+    ApplicationDialog,
+    ApplicationGroup,
+    ApplicationLog,
+    ApplicationMarquee,
+    ApplicationStatus,
+    ApplicationTextGroup,
+    ApplicationTimer,
+    Audio,
+    Blockquote,
+    Browser,
+    BusyIndicator,
+    Button,
+    Canvas,
+    Caption,
+    Cell,
+    CheckBox,
+    ColorWell,
+    Column,
+    ColumnHeader,
+    ComboBox,
+    Definition,
+    DescriptionList,
+    DescriptionListTerm,
+    DescriptionListDetail,
+    Details,
+    Directory,
+    DisclosureTriangle,
+    Div,
+    Document,
+    DocumentArticle,
+    DocumentMath,
+    DocumentNote,
+    Drawer,
+    EditableText,
+    Feed,
+    Figure,
+    Footer,
+    Footnote,
+    Form,
+    GraphicsDocument,
+    GraphicsObject,
+    GraphicsSymbol,
+    Grid,
+    GridCell,
+    Group,
+    GrowArea,
+    Heading,
+    HelpTag,
+    HorizontalRule,
+    Ignored,
+    Inline,
+    Image,
+    ImageMap,
+    ImageMapLink,
+    Incrementor,
+    Label,
+    LandmarkBanner,
+    LandmarkComplementary,
+    LandmarkContentInfo,
+    LandmarkDocRegion,
+    LandmarkMain,
+    LandmarkNavigation,
+    LandmarkRegion,
+    LandmarkSearch,
+    Legend,
+    Link,
+    List,
+    ListBox,
+    ListBoxOption,
+    ListItem,
+    ListMarker,
+    Mark,
+    MathElement,
+    Matte,
+    Menu,
+    MenuBar,
+    MenuButton,
+    MenuItem,
+    MenuItemCheckbox,
+    MenuItemRadio,
+    MenuListPopup,
+    MenuListOption,
+    Outline,
+    Paragraph,
+    PopUpButton,
+    Pre,
+    Presentational,
+    ProgressIndicator,
+    RadioButton,
+    RadioGroup,
+    RowHeader,
+    Row,
+    RowGroup,
+    RubyBase,
+    RubyBlock,
+    RubyInline,
+    RubyRun,
+    RubyText,
+    Ruler,
+    RulerMarker,
+    ScrollArea,
+    ScrollBar,
+    SearchField,
+    Sheet,
+    Slider,
+    SliderThumb,
+    SpinButton,
+    SpinButtonPart,
+    SplitGroup,
+    Splitter,
+    StaticText,
+    Summary,
+    Switch,
+    SystemWide,
+    SVGRoot,
+    SVGText,
+    SVGTSpan,
+    SVGTextPath,
+    TabGroup,
+    TabList,
+    TabPanel,
+    Tab,
+    Table,
+    TableHeaderContainer,
+    TextArea,
+    TextGroup,
+    Term,
+    Time,
+    Tree,
+    TreeGrid,
+    TreeItem,
+    TextField,
+    ToggleButton,
+    Toolbar,
+    Unknown,
+    UserInterfaceTooltip,
+    ValueIndicator,
+    Video,
+    WebApplication,
+    WebArea,
+    WebCoreLink,
+    Window,
+};
+
+class AccessibilityObjectInterface {
+public:
+    virtual ~AccessibilityObjectInterface() = default;
+
+    virtual bool isMediaControlLabel() const = 0;
+    virtual AccessibilityRole roleValue() const = 0;
+    virtual bool isAttachment() const = 0;
+};
+
+}
diff --git a/Source/WebCore/accessibility/isolatedtree/AXIsolatedTree.cpp b/Source/WebCore/accessibility/isolatedtree/AXIsolatedTree.cpp
new file mode 100644
index 0000000..4fc15d7
--- /dev/null
+++ b/Source/WebCore/accessibility/isolatedtree/AXIsolatedTree.cpp
@@ -0,0 +1,143 @@
+/*
+ * Copyright (C) 2019 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ * THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+
+#include "config.h"
+#include "AXIsolatedTree.h"
+
+#include "AXIsolatedTreeNode.h"
+#include "Page.h"
+#include <wtf/NeverDestroyed.h>
+
+namespace WebCore {
+
+static Lock s_cacheLock;
+
+static unsigned newTreeID()
+{
+    static unsigned s_currentTreeID = 0;
+    return ++s_currentTreeID;
+}
+
+HashMap<AXIsolatedTreeID, Ref<AXIsolatedTree>>& AXIsolatedTree::treePageCache()
+{
+    static NeverDestroyed<HashMap<AXIsolatedTreeID, Ref<AXIsolatedTree>>> map;
+    return map;
+}
+
+HashMap<AXIsolatedTreeID, Ref<AXIsolatedTree>>& AXIsolatedTree::treeIDCache()
+{
+    static NeverDestroyed<HashMap<AXIsolatedTreeID, Ref<AXIsolatedTree>>> map;
+    return map;
+}
+
+AXIsolatedTree::AXIsolatedTree()
+    : m_treeID(newTreeID())
+{
+}
+
+AXIsolatedTree::~AXIsolatedTree() = default;
+
+Ref<AXIsolatedTree> AXIsolatedTree::create()
+{
+    ASSERT(isMainThread());
+    return adoptRef(*new AXIsolatedTree());
+}
+
+RefPtr<AXIsolatedTree> AXIsolatedTree::treeForID(AXIsolatedTreeID treeID)
+{
+    return treeIDCache().get(treeID);
+}
+
+Ref<AXIsolatedTree> AXIsolatedTree::createTreeForPageID(uint64_t pageID)
+{
+    LockHolder locker(s_cacheLock);
+
+    auto newTree = AXIsolatedTree::create();
+    treePageCache().set(pageID, newTree.copyRef());
+    treeIDCache().set(newTree->treeIdentifier(), newTree.copyRef());
+    return newTree;
+}
+
+RefPtr<AXIsolatedTree> AXIsolatedTree::treeForPageID(uint64_t pageID)
+{
+    LockHolder locker(s_cacheLock);
+
+    if (auto tree = treePageCache().get(pageID))
+        return makeRefPtr(tree);
+
+    return nullptr;
+}
+
+RefPtr<AXIsolatedTreeNode> AXIsolatedTree::nodeForID(AXID axID) const
+{
+    ASSERT(!isMainThread());
+    if (!axID)
+        return nullptr;
+    return m_readerThreadNodeMap.get(axID);
+}
+
+RefPtr<AXIsolatedTreeNode> AXIsolatedTree::rootNode()
+{
+    return nodeForID(m_rootNodeID);
+}
+
+void AXIsolatedTree::removeNode(AXID axID)
+{
+    LockHolder locker(m_changeLogLock);
+    m_pendingRemovals.append(axID);
+}
+
+void AXIsolatedTree::appendNodeChanges(Vector<Ref<AXIsolatedTreeNode>>& log)
+{
+    LockHolder locker(m_changeLogLock);
+    for (auto& node : log)
+        m_pendingAppends.append(node.copyRef());
+}
+
+void AXIsolatedTree::applyPendingChanges()
+{
+    RELEASE_ASSERT(!isMainThread());
+
+    LockHolder locker(m_changeLogLock);
+    Vector<Ref<AXIsolatedTreeNode>> appendCopy;
+    std::swap(appendCopy, m_pendingAppends);
+    Vector<AXID> removeCopy({ WTFMove(m_pendingRemovals) });
+    locker.unlockEarly();
+
+    for (auto& item : appendCopy) {
+        m_readerThreadNodeMap.add(item->identifier(), item.copyRef());
+        if (item->isRootNode())
+            m_rootNodeID = item->identifier();
+    }
+
+    for (auto item : removeCopy)
+        m_readerThreadNodeMap.remove(item);
+}
+    
+} // namespace WebCore
+
+#endif // ENABLE(ACCESSIBILITY_ISOLATED_TREE)
diff --git a/Source/WebCore/accessibility/isolatedtree/AXIsolatedTree.h b/Source/WebCore/accessibility/isolatedtree/AXIsolatedTree.h
new file mode 100644
index 0000000..971f9ea9
--- /dev/null
+++ b/Source/WebCore/accessibility/isolatedtree/AXIsolatedTree.h
@@ -0,0 +1,85 @@
+/*
+ * Copyright (C) 2019 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+
+#include "AXIsolatedTreeNode.h"
+#include <wtf/HashMap.h>
+#include <wtf/RefPtr.h>
+#include <wtf/ThreadSafeRefCounted.h>
+
+namespace WebCore {
+
+class Page;
+
+typedef unsigned AXIsolatedTreeID;
+    
+class AXIsolatedTree : public ThreadSafeRefCounted<AXIsolatedTree> {
+    WTF_MAKE_NONCOPYABLE(AXIsolatedTree); WTF_MAKE_FAST_ALLOCATED;
+
+public:
+    static Ref<AXIsolatedTree> create();
+    virtual ~AXIsolatedTree();
+
+    // Creation must happen on main thread.
+    static Ref<AXIsolatedTree> createTreeForPageID(uint64_t pageID);
+    WEBCORE_EXPORT static RefPtr<AXIsolatedTree> treeForPageID(uint64_t pageID);
+    WEBCORE_EXPORT static RefPtr<AXIsolatedTree> treeForID(AXIsolatedTreeID);
+
+    WEBCORE_EXPORT RefPtr<AXIsolatedTreeNode> rootNode();
+    RefPtr<AXIsolatedTreeNode> nodeForID(AXID) const;
+
+    // Call on main thread
+    void appendNodeChanges(Vector<Ref<AXIsolatedTreeNode>>&);
+    void removeNode(AXID);
+
+    // Call on AX thread
+    WEBCORE_EXPORT void applyPendingChanges();
+
+    AXIsolatedTreeID treeIdentifier() const { return m_treeID; }
+
+private:
+    AXIsolatedTree();
+
+    static HashMap<AXIsolatedTreeID, Ref<AXIsolatedTree>>& treeIDCache();
+    static HashMap<AXIsolatedTreeID, Ref<AXIsolatedTree>>& treePageCache();
+    
+    // Only access on AX thread requesting data.
+    HashMap<AXID, Ref<AXIsolatedTreeNode>> m_readerThreadNodeMap;
+
+    // Written to by main thread under lock, accessed and applied by AX thread.
+    Vector<Ref<AXIsolatedTreeNode>> m_pendingAppends;
+    Vector<AXID> m_pendingRemovals;
+    Lock m_changeLogLock;
+
+    AXIsolatedTreeID m_treeID;
+    AXID m_rootNodeID;
+};
+
+} // namespace WebCore
+
+#endif
diff --git a/Source/WebCore/accessibility/isolatedtree/AXIsolatedTreeNode.cpp b/Source/WebCore/accessibility/isolatedtree/AXIsolatedTreeNode.cpp
new file mode 100644
index 0000000..12bda1f
--- /dev/null
+++ b/Source/WebCore/accessibility/isolatedtree/AXIsolatedTreeNode.cpp
@@ -0,0 +1,129 @@
+/*
+ * Copyright (C) 2019 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ * THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+
+#include "config.h"
+#include "AXIsolatedTreeNode.h"
+
+#include "AccessibilityObject.h"
+
+namespace WebCore {
+
+AXIsolatedTreeNode::AXIsolatedTreeNode(const AccessibilityObject& object)
+    : m_identifier(object.axObjectID())
+    , m_initialized(false)
+{
+    ASSERT(isMainThread());
+    initializeAttributeData(object);
+#if !ASSERT_DISABLED
+    m_initialized = true;
+#endif
+}
+
+Ref<AXIsolatedTreeNode> AXIsolatedTreeNode::create(const AccessibilityObject& object)
+{
+    return adoptRef(*new AXIsolatedTreeNode(object));
+}
+
+AXIsolatedTreeNode::~AXIsolatedTreeNode() = default;
+
+void AXIsolatedTreeNode::initializeAttributeData(const AccessibilityObject& object)
+{
+    setProperty(AXPropertyName::RoleValue, static_cast<int>(object.roleValue()));
+    setProperty(AXPropertyName::IsAttachment, object.isAttachment());
+    setProperty(AXPropertyName::IsMediaControlLabel, object.isMediaControlLabel());
+}
+
+void AXIsolatedTreeNode::setProperty(AXPropertyName propertyName, AttributeValueVariant&& value, bool shouldRemove)
+{
+    ASSERT(!m_initialized);
+    ASSERT(isMainThread());
+
+    if (shouldRemove)
+        m_attributeMap.remove(propertyName);
+    else
+        m_attributeMap.set(propertyName, value);
+}
+
+void AXIsolatedTreeNode::appendChild(AXID axID)
+{
+    ASSERT(isMainThread());
+    m_children.append(axID);
+}
+
+double AXIsolatedTreeNode::doubleAttributeValue(AXPropertyName propertyName) const
+{
+    auto value = m_attributeMap.get(propertyName);
+    return WTF::switchOn(value, [&] (double& typedValue) {
+        return typedValue;
+    }, [&] (auto&) {
+        return 0.0;
+    });
+}
+
+unsigned AXIsolatedTreeNode::unsignedAttributeValue(AXPropertyName propertyName) const
+{
+    auto value = m_attributeMap.get(propertyName);
+    return WTF::switchOn(value, [&] (unsigned& typedValue) {
+        return typedValue;
+    }, [&] (auto&) {
+        return 0;
+    });
+}
+
+bool AXIsolatedTreeNode::boolAttributeValue(AXPropertyName propertyName) const
+{
+    auto value = m_attributeMap.get(propertyName);
+    return WTF::switchOn(value, [&] (bool& typedValue) {
+        return typedValue;
+    }, [&] (auto&) {
+        return false;
+    });
+}
+
+const String& AXIsolatedTreeNode::stringAttributeValue(AXPropertyName propertyName) const
+{
+    auto value = m_attributeMap.get(propertyName);
+    return WTF::switchOn(value, [&] (String& typedValue) {
+        return typedValue;
+    }, [&] (auto&) {
+        return nullAtom();
+    });
+}
+
+int AXIsolatedTreeNode::intAttributeValue(AXPropertyName propertyName) const
+{
+    auto value = m_attributeMap.get(propertyName);
+    return WTF::switchOn(value, [&] (int& typedValue) {
+        return typedValue;
+    }, [&] (auto&) {
+        return 0;
+    });
+}
+    
+} // namespace WebCore
+
+#endif // ENABLE((ACCESSIBILITY_ISOLATED_TREE)
diff --git a/Source/WebCore/accessibility/isolatedtree/AXIsolatedTreeNode.h b/Source/WebCore/accessibility/isolatedtree/AXIsolatedTreeNode.h
new file mode 100644
index 0000000..dd3c34f
--- /dev/null
+++ b/Source/WebCore/accessibility/isolatedtree/AXIsolatedTreeNode.h
@@ -0,0 +1,120 @@
+/*
+* Copyright (C) 2019 Apple Inc. All rights reserved.
+*
+* Redistribution and use in source and binary forms, with or without
+* modification, are permitted provided that the following conditions
+* are met:
+* 1. Redistributions of source code must retain the above copyright
+*    notice, this list of conditions and the following disclaimer.
+* 2. Redistributions in binary form must reproduce the above copyright
+*    notice, this list of conditions and the following disclaimer in the
+*    documentation and/or other materials provided with the distribution.
+*
+* THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
+* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
+* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+* THE POSSIBILITY OF SUCH DAMAGE.
+*/
+
+#pragma once
+
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+
+#include "AccessibilityObjectInterface.h"
+#include <wtf/Forward.h>
+#include <wtf/HashMap.h>
+#include <wtf/RefPtr.h>
+#include <wtf/RetainPtr.h>
+#include <wtf/ThreadSafeRefCounted.h>
+#include <wtf/Variant.h>
+#include <wtf/Vector.h>
+#include <wtf/WeakPtr.h>
+
+#if PLATFORM(COCOA)
+OBJC_CLASS WebAccessibilityObjectWrapper;
+#endif
+
+namespace WebCore {
+
+class AccessibilityObject;
+
+class AXIsolatedTreeNode final : public AccessibilityObjectInterface, public ThreadSafeRefCounted<AXIsolatedTreeNode>, public CanMakeWeakPtr<AXIsolatedTreeNode> {
+
+public:
+    enum class AXPropertyName : uint8_t {
+        None = 0,
+        RoleValue = 1,
+        IsAttachment,
+        IsMediaControlLabel,
+    };
+
+    static Ref<AXIsolatedTreeNode> create(const AccessibilityObject&);
+    virtual ~AXIsolatedTreeNode();
+
+    AXID identifier() const { return m_identifier; }
+
+    bool isRootNode() const { return m_isRootNode; }
+    void setIsRootNode(bool value)
+    {
+        ASSERT(isMainThread());
+        m_isRootNode = value;
+    }
+
+    void setParent(AXID parent)
+    {
+        ASSERT(isMainThread());
+        m_parent = parent;
+    }
+    AXID parent() const { return m_parent; }
+
+    void appendChild(AXID);
+    const Vector<AXID>& children() const { return m_children; };
+
+#if PLATFORM(COCOA)
+    WebAccessibilityObjectWrapper* wrapper() const { return m_wrapper.get(); }
+    void setWrapper(WebAccessibilityObjectWrapper* wrapper) { m_wrapper = wrapper; }
+#endif
+
+protected:
+    AXIsolatedTreeNode() = default;
+
+private:
+    AXIsolatedTreeNode(const AccessibilityObject&);
+    void initializeAttributeData(const AccessibilityObject&);
+
+    bool isMediaControlLabel() const override { return boolAttributeValue(AXPropertyName::IsMediaControlLabel); }
+    bool isAttachment() const override { return boolAttributeValue(AXPropertyName::IsAttachment); }
+    AccessibilityRole roleValue() const override { return static_cast<AccessibilityRole>(intAttributeValue(AXPropertyName::RoleValue)); }
+
+    using AttributeValueVariant = Variant<std::nullptr_t, String, bool, int, unsigned, double>;
+    void setProperty(AXPropertyName, AttributeValueVariant&&, bool shouldRemove = false);
+
+    bool boolAttributeValue(AXPropertyName) const;
+    const String& stringAttributeValue(AXPropertyName) const;
+    int intAttributeValue(AXPropertyName) const;
+    unsigned unsignedAttributeValue(AXPropertyName) const;
+    double doubleAttributeValue(AXPropertyName) const;
+
+    AXID m_parent;
+    AXID m_identifier;
+    bool m_isRootNode;
+    bool m_initialized;
+    Vector<AXID> m_children;
+
+#if PLATFORM(COCOA)
+    RetainPtr<WebAccessibilityObjectWrapper> m_wrapper;
+#endif
+
+    HashMap<AXPropertyName, AttributeValueVariant, WTF::IntHash<AXPropertyName>, WTF::StrongEnumHashTraits<AXPropertyName>> m_attributeMap;
+};
+
+} // namespace WebCore
+
+#endif // ENABLE((ACCESSIBILITY_ISOLATED_TREE))
diff --git a/Source/WebCore/accessibility/mac/AXObjectCacheMac.mm b/Source/WebCore/accessibility/mac/AXObjectCacheMac.mm
index f754976..2fd9d9a 100644
--- a/Source/WebCore/accessibility/mac/AXObjectCacheMac.mm
+++ b/Source/WebCore/accessibility/mac/AXObjectCacheMac.mm
@@ -28,6 +28,7 @@
 
 #if HAVE(ACCESSIBILITY) && PLATFORM(MAC)
 
+#import "AXIsolatedTreeNode.h"
 #import "AccessibilityObject.h"
 #import "AccessibilityTable.h"
 #import "RenderObject.h"
@@ -237,6 +238,14 @@
     obj->setWrapper(nullptr);
 }
 
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+void AXObjectCache::associateIsolatedTreeNode(AccessibilityObject& object, AXIsolatedTreeNode& node, AXIsolatedTreeID treeID)
+{
+    object.wrapper().isolatedTreeIdentifier = treeID;
+    node.setWrapper(object.wrapper());
+}
+#endif
+
 void AXObjectCache::attachWrapper(AccessibilityObject* obj)
 {
     RetainPtr<WebAccessibilityObjectWrapper> wrapper = adoptNS([[WebAccessibilityObjectWrapper alloc] initWithAccessibilityObject:obj]);
diff --git a/Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperBase.h b/Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperBase.h
index c31cab8..0cc4104 100644
--- a/Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperBase.h
+++ b/Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperBase.h
@@ -29,11 +29,18 @@
 #ifndef WebAccessibilityObjectWrapperBase_h
 #define WebAccessibilityObjectWrapperBase_h
 
+#include "AXIsolatedTree.h"
 #include "AccessibilityObject.h"
 #include <CoreGraphics/CoreGraphics.h>
+#include <wtf/RefPtr.h>
+#include <wtf/Variant.h>
+#include <wtf/WeakPtr.h>
 
 namespace WebCore {
 class AccessibilityObject;
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+class AXIsolatedTreeNode;
+#endif
 struct AccessibilitySearchCriteria;
 class IntRect;
 class FloatPoint;
@@ -44,13 +51,26 @@
 
 @interface WebAccessibilityObjectWrapperBase : NSObject {
     WebCore::AccessibilityObject* m_object;
+    WebCore::AXID _identifier;
 }
  
 - (id)initWithAccessibilityObject:(WebCore::AccessibilityObject*)axObject;
+
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+@property (nonatomic, readonly) RefPtr<WebCore::AXIsolatedTreeNode> isolatedTreeNode;
+@property (nonatomic, assign) WebCore::AXIsolatedTreeID isolatedTreeIdentifier;
+#endif
+
 - (void)detach;
+
+@property (nonatomic, assign) WebCore::AXID identifier;
+
 - (WebCore::AccessibilityObject*)accessibilityObject;
 - (BOOL)updateObjectBackingStore;
 
+// This can be either an AccessibilityObject or an AXIsolatedTreeNode
+- (WebCore::AccessibilityObjectInterface*)axBackingObject;
+
 // These are pre-fixed with base so that AppKit does not end up calling into these directly (bypassing safety checks).
 - (NSString *)baseAccessibilityTitle;
 - (NSString *)baseAccessibilityDescription;
diff --git a/Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperBase.mm b/Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperBase.mm
index 7ad8510..44d3d69 100644
--- a/Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperBase.mm
+++ b/Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperBase.mm
@@ -31,10 +31,13 @@
 
 #if HAVE(ACCESSIBILITY)
 
+#import "AXIsolatedTree.h"
+#import "AXIsolatedTreeNode.h"
 #import "AXObjectCache.h"
 #import "AccessibilityARIAGridRow.h"
 #import "AccessibilityList.h"
 #import "AccessibilityListBox.h"
+#import "AccessibilityObjectInterface.h"
 #import "AccessibilityRenderObject.h"
 #import "AccessibilityScrollView.h"
 #import "AccessibilitySpinButton.h"
@@ -270,22 +273,47 @@
 
 @implementation WebAccessibilityObjectWrapperBase
 
+@synthesize identifier=_identifier;
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+@synthesize isolatedTreeIdentifier=_isolatedTreeIdentifier;
+#endif
+
 - (id)initWithAccessibilityObject:(AccessibilityObject*)axObject
 {
     if (!(self = [super init]))
         return nil;
 
     m_object = axObject;
+    _identifier = m_object->axObjectID();
+
     return self;
 }
 
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+- (RefPtr<WebCore::AXIsolatedTreeNode>)isolatedTreeNode
+{
+    RELEASE_ASSERT(!isMainThread());
+    return AXIsolatedTree::treeForID(_isolatedTreeIdentifier)->nodeForID(_identifier);
+}
+#endif
+
 - (void)detach
 {
     m_object = nullptr;
+    _identifier = 0;
+
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+    _isolatedTreeIdentifier = 0;
+#endif
 }
 
 - (BOOL)updateObjectBackingStore
 {
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+    RELEASE_ASSERT(!isMainThread());
+    AXIsolatedTree::treeForID(self.isolatedTreeIdentifier)->applyPendingChanges();
+    return _identifier;
+#else
     // Calling updateBackingStore() can invalidate this element so self must be retained.
     // If it does become invalidated, m_object will be nil.
     CFRetain((__bridge CFTypeRef)self);
@@ -299,6 +327,7 @@
         return NO;
     
     return YES;
+#endif
 }
 
 - (id)attachmentView
@@ -360,11 +389,21 @@
     return [NSString string];
 }
 
+#define _axBackingObject self.axBackingObject
+- (WebCore::AccessibilityObjectInterface*)axBackingObject
+{
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+    return self.isolatedTreeNode.get();
+#else
+    return m_object;
+#endif
+}
+
 - (NSString *)baseAccessibilityDescription
 {
     // Static text objects should not have a description. Its content is communicated in its AXValue.
     // One exception is the media control labels that have a value and a description. Those are set programatically.
-    if (m_object->roleValue() == AccessibilityRole::StaticText && !m_object->isMediaControlLabel())
+    if (_axBackingObject->roleValue() == AccessibilityRole::StaticText && !_axBackingObject->isMediaControlLabel())
         return [NSString string];
     
     Vector<AccessibilityText> textOrder;
diff --git a/Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperMac.mm b/Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperMac.mm
index 370597f..ea83ea1 100644
--- a/Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperMac.mm
+++ b/Source/WebCore/accessibility/mac/WebAccessibilityObjectWrapperMac.mm
@@ -416,6 +416,8 @@
 #define NSAccessibilityRelativeFrameAttribute @"AXRelativeFrame"
 #endif
 
+#define _axBackingObject self.axBackingObject
+
 extern "C" AXUIElementRef NSAccessibilityCreateAXUIElementRef(id element);
 
 @implementation WebAccessibilityObjectWrapper
@@ -1970,10 +1972,10 @@
 - (NSString*)role
 {
     ALLOW_DEPRECATED_DECLARATIONS_BEGIN
-    if (m_object->isAttachment())
+    if (_axBackingObject->isAttachment())
         return [[self attachmentView] accessibilityAttributeValue:NSAccessibilityRoleAttribute];
     ALLOW_DEPRECATED_DECLARATIONS_END
-    AccessibilityRole role = m_object->roleValue();
+    AccessibilityRole role = _axBackingObject->roleValue();
 
     if (role == AccessibilityRole::Label && is<AccessibilityLabel>(*m_object) && downcast<AccessibilityLabel>(*m_object).containsOnlyStaticText())
         role = AccessibilityRole::StaticText;
@@ -2000,7 +2002,7 @@
     if (m_object->isSearchField())
         return NSAccessibilitySearchFieldSubrole;
     
-    if (m_object->isAttachment()) {
+    if (_axBackingObject->isAttachment()) {
         NSView* attachView = [self attachmentView];
         if ([[attachView accessibilityAttributeNames] containsObject:NSAccessibilitySubroleAttribute])
             return [attachView accessibilityAttributeValue:NSAccessibilitySubroleAttribute];
@@ -2211,7 +2213,7 @@
 
     ALLOW_DEPRECATED_DECLARATIONS_BEGIN
     // attachments have the AXImage role, but a different subrole
-    if (m_object->isAttachment())
+    if (_axBackingObject->isAttachment())
         return [[self attachmentView] accessibilityAttributeValue:NSAccessibilityRoleDescriptionAttribute];
     ALLOW_DEPRECATED_DECLARATIONS_END
 
@@ -2568,7 +2570,7 @@
         return [NSNumber numberWithBool: m_object->isVisited()];
     
     if ([attributeName isEqualToString: NSAccessibilityTitleAttribute]) {
-        if (m_object->isAttachment()) {
+        if (_axBackingObject->isAttachment()) {
             if ([[[self attachmentView] accessibilityAttributeNames] containsObject:NSAccessibilityTitleAttribute])
                 return [[self attachmentView] accessibilityAttributeValue:NSAccessibilityTitleAttribute];
         }
@@ -2585,7 +2587,7 @@
     }
     
     if ([attributeName isEqualToString: NSAccessibilityDescriptionAttribute]) {
-        if (m_object->isAttachment()) {
+        if (_axBackingObject->isAttachment()) {
             if ([[[self attachmentView] accessibilityAttributeNames] containsObject:NSAccessibilityDescriptionAttribute])
                 return [[self attachmentView] accessibilityAttributeValue:NSAccessibilityDescriptionAttribute];
         }
@@ -2593,7 +2595,7 @@
     }
     
     if ([attributeName isEqualToString: NSAccessibilityValueAttribute]) {
-        if (m_object->isAttachment()) {
+        if (_axBackingObject->isAttachment()) {
             if ([[[self attachmentView] accessibilityAttributeNames] containsObject:NSAccessibilityValueAttribute])
                 return [[self attachmentView] accessibilityAttributeValue:NSAccessibilityValueAttribute];
         }
diff --git a/Source/WebCore/dom/Document.cpp b/Source/WebCore/dom/Document.cpp
index f86b36f..6d443e7 100644
--- a/Source/WebCore/dom/Document.cpp
+++ b/Source/WebCore/dom/Document.cpp
@@ -8217,6 +8217,11 @@
 
     return *m_logger;
 }
+    
+Optional<uint64_t> Document::pageID() const
+{
+    return m_frame->loader().client().pageID();
+}
 
 void Document::hasStorageAccess(Ref<DeferredPromise>&& promise)
 {
diff --git a/Source/WebCore/dom/Document.h b/Source/WebCore/dom/Document.h
index bb5c1c0..30f0ee6 100644
--- a/Source/WebCore/dom/Document.h
+++ b/Source/WebCore/dom/Document.h
@@ -648,6 +648,7 @@
     WEBCORE_EXPORT AXObjectCache* axObjectCache() const;
     void clearAXObjectCache();
 
+    Optional<uint64_t> pageID() const;
     // to get visually ordered hebrew and arabic pages right
     void setVisuallyOrdered();
     bool visuallyOrdered() const { return m_visuallyOrdered; }
diff --git a/Source/WebKit/ChangeLog b/Source/WebKit/ChangeLog
index 03a1013..10d9d2c 100644
--- a/Source/WebKit/ChangeLog
+++ b/Source/WebKit/ChangeLog
@@ -1,3 +1,15 @@
+2019-01-27  Chris Fleizach  <cfleizach@apple.com>
+
+        AX: Introduce a static accessibility tree
+        https://bugs.webkit.org/show_bug.cgi?id=193348
+        <rdar://problem/47203295>
+
+        Reviewed by Ryosuke Niwa.
+
+        * Configurations/FeatureDefines.xcconfig:
+        * WebProcess/WebPage/mac/WKAccessibilityWebPageObjectBase.mm:
+        (-[WKAccessibilityWebPageObjectBase accessibilityRootObjectWrapper]):
+
 2019-01-26  Simon Fraser  <simon.fraser@apple.com>
 
         Allow scrolling tree nodes to exist in a detached state
diff --git a/Source/WebKit/Configurations/FeatureDefines.xcconfig b/Source/WebKit/Configurations/FeatureDefines.xcconfig
index 9122a25..230623e 100644
--- a/Source/WebKit/Configurations/FeatureDefines.xcconfig
+++ b/Source/WebKit/Configurations/FeatureDefines.xcconfig
@@ -40,6 +40,8 @@
 ENABLE_ACCELERATED_OVERFLOW_SCROLLING = $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING_$(WK_COCOA_TOUCH));
 ENABLE_ACCELERATED_OVERFLOW_SCROLLING_cocoatouch = ENABLE_ACCELERATED_OVERFLOW_SCROLLING;
 
+ENABLE_ACCESSIBILITY_ISOLATED_TREE = ;
+
 ENABLE_APPLE_PAY = $(ENABLE_APPLE_PAY_$(WK_PLATFORM_NAME));
 ENABLE_APPLE_PAY_iphoneos = ENABLE_APPLE_PAY;
 ENABLE_APPLE_PAY_iphonesimulator = ENABLE_APPLE_PAY;
@@ -407,4 +409,4 @@
 
 ENABLE_XSLT = ENABLE_XSLT;
 
-FEATURE_DEFINES = $(ENABLE_3D_TRANSFORMS) $(ENABLE_ACCELERATED_2D_CANVAS) $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING) $(ENABLE_APPLE_PAY) $(ENABLE_APPLE_PAY_SESSION_V3) $(ENABLE_APPLE_PAY_SESSION_V4) $(ENABLE_APPLICATION_MANIFEST) $(ENABLE_ATTACHMENT_ELEMENT) $(ENABLE_AVF_CAPTIONS) $(ENABLE_C_LOOP) $(ENABLE_CACHE_PARTITIONING) $(ENABLE_CHANNEL_MESSAGING) $(ENABLE_CONTENT_FILTERING) $(ENABLE_CSS3_TEXT) $(ENABLE_CSS_BOX_DECORATION_BREAK) $(ENABLE_CSS_COMPOSITING) $(ENABLE_CSS_DEVICE_ADAPTATION) $(ENABLE_CSS_IMAGE_ORIENTATION) $(ENABLE_CSS_IMAGE_RESOLUTION) $(ENABLE_CSS_PAINTING_API) $(ENABLE_CSS_SCROLL_SNAP) $(ENABLE_CSS_SELECTORS_LEVEL4) $(ENABLE_CSS_TRAILING_WORD) $(ENABLE_CSS_TYPED_OM) $(ENABLE_CURSOR_VISIBILITY) $(ENABLE_CUSTOM_SCHEME_HANDLER) $(ENABLE_CSS_CONIC_GRADIENTS) $(ENABLE_DARK_MODE_CSS) $(ENABLE_DASHBOARD_SUPPORT) $(ENABLE_DATA_INTERACTION) $(ENABLE_DATACUE_VALUE) $(ENABLE_DATALIST_ELEMENT) $(ENABLE_DEVICE_ORIENTATION) $(ENABLE_DRAG_SUPPORT) $(ENABLE_ENCRYPTED_MEDIA) $(ENABLE_EXPERIMENTAL_FEATURES) $(ENABLE_FAST_JIT_PERMISSIONS) $(ENABLE_FILTERS_LEVEL_2) $(ENABLE_FTL_JIT) $(ENABLE_FULLSCREEN_API) $(ENABLE_GAMEPAD) $(ENABLE_GEOLOCATION) $(ENABLE_ICONDATABASE) $(ENABLE_INDEXED_DATABASE) $(ENABLE_INDEXED_DATABASE_IN_WORKERS) $(ENABLE_INPUT_TYPE_COLOR) $(ENABLE_INPUT_TYPE_DATE) $(ENABLE_INPUT_TYPE_DATETIME_INCOMPLETE) $(ENABLE_INPUT_TYPE_DATETIMELOCAL) $(ENABLE_INPUT_TYPE_MONTH) $(ENABLE_INPUT_TYPE_TIME) $(ENABLE_INPUT_TYPE_WEEK) $(ENABLE_INTERSECTION_OBSERVER) $(ENABLE_INTL) $(ENABLE_IOS_GESTURE_EVENTS) $(ENABLE_IOS_TOUCH_EVENTS) $(ENABLE_JIT) $(ENABLE_KEYBOARD_CODE_ATTRIBUTE) $(ENABLE_KEYBOARD_KEY_ATTRIBUTE) $(ENABLE_LAYOUT_FORMATTING_CONTEXT) $(ENABLE_LEGACY_CSS_VENDOR_PREFIXES) $(ENABLE_LEGACY_CUSTOM_PROTOCOL_MANAGER) $(ENABLE_LEGACY_ENCRYPTED_MEDIA) $(ENABLE_LETTERPRESS) $(ENABLE_MAC_GESTURE_EVENTS) $(ENABLE_MAC_VIDEO_TOOLBOX) $(ENABLE_MATHML) $(ENABLE_MEDIA_CAPTURE) $(ENABLE_MEDIA_CONTROLS_SCRIPT) $(ENABLE_MEDIA_SESSION) $(ENABLE_MEDIA_SOURCE) $(ENABLE_MEDIA_STATISTICS) $(ENABLE_MEDIA_STREAM) $(ENABLE_METER_ELEMENT) $(ENABLE_MHTML) $(ENABLE_MOUSE_CURSOR_SCALE) $(ENABLE_NAVIGATOR_CONTENT_UTILS) $(ENABLE_NAVIGATOR_STANDALONE) $(ENABLE_NOTIFICATIONS) $(ENABLE_PAYMENT_REQUEST) $(ENABLE_PDFKIT_PLUGIN) $(ENABLE_POINTER_EVENTS) $(ENABLE_POINTER_LOCK) $(ENABLE_PUBLIC_SUFFIX_LIST) $(ENABLE_QUOTA) $(ENABLE_REMOTE_INSPECTOR) $(ENABLE_RESOLUTION_MEDIA_QUERY) $(ENABLE_RESOURCE_LOAD_STATISTICS) $(ENABLE_RESOURCE_USAGE) $(ENABLE_RUBBER_BANDING) $(ENABLE_SERVICE_CONTROLS) $(ENABLE_SERVICE_WORKER) $(ENABLE_SPEECH_SYNTHESIS) $(ENABLE_STREAMS_API) $(ENABLE_WEB_CRYPTO) $(ENABLE_SVG_FONTS) $(ENABLE_TELEPHONE_NUMBER_DETECTION) $(ENABLE_TEXT_AUTOSIZING) $(ENABLE_TOUCH_EVENTS) $(ENABLE_USER_MESSAGE_HANDLERS) $(ENABLE_USERSELECT_ALL) $(ENABLE_VARIATION_FONTS) $(ENABLE_VIDEO) $(ENABLE_VIDEO_PRESENTATION_MODE) $(ENABLE_VIDEO_TRACK) $(ENABLE_VIDEO_USES_ELEMENT_FULLSCREEN) $(ENABLE_WEB_API_STATISTICS) $(ENABLE_WEB_AUDIO) $(ENABLE_WEB_AUTHN) $(ENABLE_WEB_RTC) $(ENABLE_WEBASSEMBLY) $(ENABLE_WEBASSEMBLY_STREAMING_API) $(ENABLE_WEBGL) $(ENABLE_WEBGL2) $(ENABLE_WEBGPU) $(ENABLE_WEBMETAL) $(ENABLE_WIRELESS_PLAYBACK_TARGET) $(ENABLE_XSLT);
+FEATURE_DEFINES = $(ENABLE_3D_TRANSFORMS) $(ENABLE_ACCELERATED_2D_CANVAS) $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING) $(ENABLE_ACCESSIBILITY_ISOLATED_TREE) $(ENABLE_APPLE_PAY) $(ENABLE_APPLE_PAY_SESSION_V3) $(ENABLE_APPLE_PAY_SESSION_V4) $(ENABLE_APPLICATION_MANIFEST) $(ENABLE_ATTACHMENT_ELEMENT) $(ENABLE_AVF_CAPTIONS) $(ENABLE_C_LOOP) $(ENABLE_CACHE_PARTITIONING) $(ENABLE_CHANNEL_MESSAGING) $(ENABLE_CONTENT_FILTERING) $(ENABLE_CSS3_TEXT) $(ENABLE_CSS_BOX_DECORATION_BREAK) $(ENABLE_CSS_COMPOSITING) $(ENABLE_CSS_DEVICE_ADAPTATION) $(ENABLE_CSS_IMAGE_ORIENTATION) $(ENABLE_CSS_IMAGE_RESOLUTION) $(ENABLE_CSS_PAINTING_API) $(ENABLE_CSS_SCROLL_SNAP) $(ENABLE_CSS_SELECTORS_LEVEL4) $(ENABLE_CSS_TRAILING_WORD) $(ENABLE_CSS_TYPED_OM) $(ENABLE_CURSOR_VISIBILITY) $(ENABLE_CUSTOM_SCHEME_HANDLER) $(ENABLE_CSS_CONIC_GRADIENTS) $(ENABLE_DARK_MODE_CSS) $(ENABLE_DASHBOARD_SUPPORT) $(ENABLE_DATA_INTERACTION) $(ENABLE_DATACUE_VALUE) $(ENABLE_DATALIST_ELEMENT) $(ENABLE_DEVICE_ORIENTATION) $(ENABLE_DRAG_SUPPORT) $(ENABLE_ENCRYPTED_MEDIA) $(ENABLE_EXPERIMENTAL_FEATURES) $(ENABLE_FAST_JIT_PERMISSIONS) $(ENABLE_FILTERS_LEVEL_2) $(ENABLE_FTL_JIT) $(ENABLE_FULLSCREEN_API) $(ENABLE_GAMEPAD) $(ENABLE_GEOLOCATION) $(ENABLE_ICONDATABASE) $(ENABLE_INDEXED_DATABASE) $(ENABLE_INDEXED_DATABASE_IN_WORKERS) $(ENABLE_INPUT_TYPE_COLOR) $(ENABLE_INPUT_TYPE_DATE) $(ENABLE_INPUT_TYPE_DATETIME_INCOMPLETE) $(ENABLE_INPUT_TYPE_DATETIMELOCAL) $(ENABLE_INPUT_TYPE_MONTH) $(ENABLE_INPUT_TYPE_TIME) $(ENABLE_INPUT_TYPE_WEEK) $(ENABLE_INTERSECTION_OBSERVER) $(ENABLE_INTL) $(ENABLE_IOS_GESTURE_EVENTS) $(ENABLE_IOS_TOUCH_EVENTS) $(ENABLE_JIT) $(ENABLE_KEYBOARD_CODE_ATTRIBUTE) $(ENABLE_KEYBOARD_KEY_ATTRIBUTE) $(ENABLE_LAYOUT_FORMATTING_CONTEXT) $(ENABLE_LEGACY_CSS_VENDOR_PREFIXES) $(ENABLE_LEGACY_CUSTOM_PROTOCOL_MANAGER) $(ENABLE_LEGACY_ENCRYPTED_MEDIA) $(ENABLE_LETTERPRESS) $(ENABLE_MAC_GESTURE_EVENTS) $(ENABLE_MAC_VIDEO_TOOLBOX) $(ENABLE_MATHML) $(ENABLE_MEDIA_CAPTURE) $(ENABLE_MEDIA_CONTROLS_SCRIPT) $(ENABLE_MEDIA_SESSION) $(ENABLE_MEDIA_SOURCE) $(ENABLE_MEDIA_STATISTICS) $(ENABLE_MEDIA_STREAM) $(ENABLE_METER_ELEMENT) $(ENABLE_MHTML) $(ENABLE_MOUSE_CURSOR_SCALE) $(ENABLE_NAVIGATOR_CONTENT_UTILS) $(ENABLE_NAVIGATOR_STANDALONE) $(ENABLE_NOTIFICATIONS) $(ENABLE_PAYMENT_REQUEST) $(ENABLE_PDFKIT_PLUGIN) $(ENABLE_POINTER_EVENTS) $(ENABLE_POINTER_LOCK) $(ENABLE_PUBLIC_SUFFIX_LIST) $(ENABLE_QUOTA) $(ENABLE_REMOTE_INSPECTOR) $(ENABLE_RESOLUTION_MEDIA_QUERY) $(ENABLE_RESOURCE_LOAD_STATISTICS) $(ENABLE_RESOURCE_USAGE) $(ENABLE_RUBBER_BANDING) $(ENABLE_SERVICE_CONTROLS) $(ENABLE_SERVICE_WORKER) $(ENABLE_SPEECH_SYNTHESIS) $(ENABLE_STREAMS_API) $(ENABLE_WEB_CRYPTO) $(ENABLE_SVG_FONTS) $(ENABLE_TELEPHONE_NUMBER_DETECTION) $(ENABLE_TEXT_AUTOSIZING) $(ENABLE_TOUCH_EVENTS) $(ENABLE_USER_MESSAGE_HANDLERS) $(ENABLE_USERSELECT_ALL) $(ENABLE_VARIATION_FONTS) $(ENABLE_VIDEO) $(ENABLE_VIDEO_PRESENTATION_MODE) $(ENABLE_VIDEO_TRACK) $(ENABLE_VIDEO_USES_ELEMENT_FULLSCREEN) $(ENABLE_WEB_API_STATISTICS) $(ENABLE_WEB_AUDIO) $(ENABLE_WEB_AUTHN) $(ENABLE_WEB_RTC) $(ENABLE_WEBASSEMBLY) $(ENABLE_WEBASSEMBLY_STREAMING_API) $(ENABLE_WEBGL) $(ENABLE_WEBGL2) $(ENABLE_WEBGPU) $(ENABLE_WEBMETAL) $(ENABLE_WIRELESS_PLAYBACK_TARGET) $(ENABLE_XSLT);
diff --git a/Source/WebKit/WebProcess/WebPage/mac/WKAccessibilityWebPageObjectBase.h b/Source/WebKit/WebProcess/WebPage/mac/WKAccessibilityWebPageObjectBase.h
index be793b7..bb47ab3 100644
--- a/Source/WebKit/WebProcess/WebPage/mac/WKAccessibilityWebPageObjectBase.h
+++ b/Source/WebKit/WebProcess/WebPage/mac/WKAccessibilityWebPageObjectBase.h
@@ -32,7 +32,9 @@
 
 @interface WKAccessibilityWebPageObjectBase : NSObject {
     WebKit::WebPage* m_page;
+    uint64_t m_pageID;
     id m_parent;
+    bool m_hasPlugin;
 }
 
 - (void)setWebPage:(WebKit::WebPage*)page;
diff --git a/Source/WebKit/WebProcess/WebPage/mac/WKAccessibilityWebPageObjectBase.mm b/Source/WebKit/WebProcess/WebPage/mac/WKAccessibilityWebPageObjectBase.mm
index 9a90506..994cbaa 100644
--- a/Source/WebKit/WebProcess/WebPage/mac/WKAccessibilityWebPageObjectBase.mm
+++ b/Source/WebKit/WebProcess/WebPage/mac/WKAccessibilityWebPageObjectBase.mm
@@ -34,6 +34,7 @@
 #import "WKSharedAPICast.h"
 #import "WKString.h"
 #import "WKStringCF.h"
+#import <WebCore/AXIsolatedTree.h>
 #import <WebCore/AXObjectCache.h>
 #import <WebCore/Document.h>
 #import <WebCore/Frame.h>
@@ -47,39 +48,93 @@
 
 @implementation WKAccessibilityWebPageObjectBase
 
+- (WebCore::AXObjectCache*)axObjectCache
+{
+    WebCore::Page* page = m_page->corePage();
+    if (!page)
+        return nil;
+    WebCore::Frame& core = page->mainFrame();
+    if (!core.document())
+        return nil;
+
+    return core.document()->axObjectCache();
+}
+
+- (id)accessibilityPluginObject
+{
+    auto retrieveBlock = [&self]() -> id {
+        id axPlugin = nil;
+        auto dispatchBlock = [&axPlugin, &self] {
+            if (self->m_page)
+                axPlugin = self->m_page->accessibilityObjectForMainFramePlugin();
+        };
+
+        if (isMainThread())
+            dispatchBlock();
+        else {
+            callOnMainThreadAndWait([&dispatchBlock] {
+                dispatchBlock();
+            });
+        }
+        return axPlugin;
+    };
+    
+    return retrieveBlock();
+}
+
 - (id)accessibilityRootObjectWrapper
 {
     if (!WebCore::AXObjectCache::accessibilityEnabled())
         WebCore::AXObjectCache::enableAccessibility();
 
-    if (!m_page)
-        return nil;
-    
-    NSObject* mainFramePluginAccessibilityObjectWrapper = m_page->accessibilityObjectForMainFramePlugin();
-    if (mainFramePluginAccessibilityObjectWrapper)
-        return mainFramePluginAccessibilityObjectWrapper;
+    if (m_hasPlugin)
+        return self.accessibilityPluginObject;
 
-    WebCore::Page* page = m_page->corePage();
-    if (!page)
-        return nil;
-    
-    WebCore::Frame& core = page->mainFrame();
-    if (!core.document())
-        return nil;
-    
-    WebCore::AXObjectCache* cache = core.document()->axObjectCache();
-    if (!cache)
-        return nil;
-    
-    if (WebCore::AccessibilityObject* root = cache->rootObject())
-        return root->wrapper();
-    
+#if ENABLE(ACCESSIBILITY_ISOLATED_TREE)
+    auto generateBlock = [&] {
+        auto dispatchBlock = [&self] {
+            if (auto cache = [self axObjectCache])
+                cache->generateIsolatedAccessibilityTree();
+        };
+
+        if (isMainThread())
+            dispatchBlock();
+        else {
+            callOnMainThreadAndWait([&dispatchBlock] {
+                dispatchBlock();
+            });
+        }
+    };
+
+    auto tree = AXIsolatedTree::treeForPageID(m_pageID);
+    if (!tree)
+        generateBlock();
+
+    if ((tree = AXIsolatedTree::treeForPageID(m_pageID))) {
+        ASSERT(!isMainThread());
+        tree->applyPendingChanges();
+        return tree->rootNode()->wrapper();
+    }
+#else
+    if (AXObjectCache* cache = [self axObjectCache]) {
+        if (WebCore::AccessibilityObject* root = cache->rootObject())
+            return root->wrapper();
+    }
+#endif
     return nil;
 }
 
 - (void)setWebPage:(WebPage*)page
 {
     m_page = page;
+    
+    if (page) {
+        m_pageID = page->pageID();
+        m_hasPlugin = page->accessibilityObjectForMainFramePlugin();
+    } else {
+        m_pageID = 0;
+        m_hasPlugin = false;
+    }
 }
 
 - (void)setRemoteParent:(id)parent
diff --git a/Source/WebKitLegacy/mac/ChangeLog b/Source/WebKitLegacy/mac/ChangeLog
index 3c6d71d..003561e 100644
--- a/Source/WebKitLegacy/mac/ChangeLog
+++ b/Source/WebKitLegacy/mac/ChangeLog
@@ -1,3 +1,13 @@
+2019-01-27  Chris Fleizach  <cfleizach@apple.com>
+
+        AX: Introduce a static accessibility tree
+        https://bugs.webkit.org/show_bug.cgi?id=193348
+        <rdar://problem/47203295>
+
+        Reviewed by Ryosuke Niwa.
+
+        * Configurations/FeatureDefines.xcconfig:
+
 2019-01-25  Keith Rollin  <krollin@apple.com>
 
         Update WebKitAdditions.xcconfig with correct order of variable definitions
diff --git a/Source/WebKitLegacy/mac/Configurations/FeatureDefines.xcconfig b/Source/WebKitLegacy/mac/Configurations/FeatureDefines.xcconfig
index 9122a25..230623e 100644
--- a/Source/WebKitLegacy/mac/Configurations/FeatureDefines.xcconfig
+++ b/Source/WebKitLegacy/mac/Configurations/FeatureDefines.xcconfig
@@ -40,6 +40,8 @@
 ENABLE_ACCELERATED_OVERFLOW_SCROLLING = $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING_$(WK_COCOA_TOUCH));
 ENABLE_ACCELERATED_OVERFLOW_SCROLLING_cocoatouch = ENABLE_ACCELERATED_OVERFLOW_SCROLLING;
 
+ENABLE_ACCESSIBILITY_ISOLATED_TREE = ;
+
 ENABLE_APPLE_PAY = $(ENABLE_APPLE_PAY_$(WK_PLATFORM_NAME));
 ENABLE_APPLE_PAY_iphoneos = ENABLE_APPLE_PAY;
 ENABLE_APPLE_PAY_iphonesimulator = ENABLE_APPLE_PAY;
@@ -407,4 +409,4 @@
 
 ENABLE_XSLT = ENABLE_XSLT;
 
-FEATURE_DEFINES = $(ENABLE_3D_TRANSFORMS) $(ENABLE_ACCELERATED_2D_CANVAS) $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING) $(ENABLE_APPLE_PAY) $(ENABLE_APPLE_PAY_SESSION_V3) $(ENABLE_APPLE_PAY_SESSION_V4) $(ENABLE_APPLICATION_MANIFEST) $(ENABLE_ATTACHMENT_ELEMENT) $(ENABLE_AVF_CAPTIONS) $(ENABLE_C_LOOP) $(ENABLE_CACHE_PARTITIONING) $(ENABLE_CHANNEL_MESSAGING) $(ENABLE_CONTENT_FILTERING) $(ENABLE_CSS3_TEXT) $(ENABLE_CSS_BOX_DECORATION_BREAK) $(ENABLE_CSS_COMPOSITING) $(ENABLE_CSS_DEVICE_ADAPTATION) $(ENABLE_CSS_IMAGE_ORIENTATION) $(ENABLE_CSS_IMAGE_RESOLUTION) $(ENABLE_CSS_PAINTING_API) $(ENABLE_CSS_SCROLL_SNAP) $(ENABLE_CSS_SELECTORS_LEVEL4) $(ENABLE_CSS_TRAILING_WORD) $(ENABLE_CSS_TYPED_OM) $(ENABLE_CURSOR_VISIBILITY) $(ENABLE_CUSTOM_SCHEME_HANDLER) $(ENABLE_CSS_CONIC_GRADIENTS) $(ENABLE_DARK_MODE_CSS) $(ENABLE_DASHBOARD_SUPPORT) $(ENABLE_DATA_INTERACTION) $(ENABLE_DATACUE_VALUE) $(ENABLE_DATALIST_ELEMENT) $(ENABLE_DEVICE_ORIENTATION) $(ENABLE_DRAG_SUPPORT) $(ENABLE_ENCRYPTED_MEDIA) $(ENABLE_EXPERIMENTAL_FEATURES) $(ENABLE_FAST_JIT_PERMISSIONS) $(ENABLE_FILTERS_LEVEL_2) $(ENABLE_FTL_JIT) $(ENABLE_FULLSCREEN_API) $(ENABLE_GAMEPAD) $(ENABLE_GEOLOCATION) $(ENABLE_ICONDATABASE) $(ENABLE_INDEXED_DATABASE) $(ENABLE_INDEXED_DATABASE_IN_WORKERS) $(ENABLE_INPUT_TYPE_COLOR) $(ENABLE_INPUT_TYPE_DATE) $(ENABLE_INPUT_TYPE_DATETIME_INCOMPLETE) $(ENABLE_INPUT_TYPE_DATETIMELOCAL) $(ENABLE_INPUT_TYPE_MONTH) $(ENABLE_INPUT_TYPE_TIME) $(ENABLE_INPUT_TYPE_WEEK) $(ENABLE_INTERSECTION_OBSERVER) $(ENABLE_INTL) $(ENABLE_IOS_GESTURE_EVENTS) $(ENABLE_IOS_TOUCH_EVENTS) $(ENABLE_JIT) $(ENABLE_KEYBOARD_CODE_ATTRIBUTE) $(ENABLE_KEYBOARD_KEY_ATTRIBUTE) $(ENABLE_LAYOUT_FORMATTING_CONTEXT) $(ENABLE_LEGACY_CSS_VENDOR_PREFIXES) $(ENABLE_LEGACY_CUSTOM_PROTOCOL_MANAGER) $(ENABLE_LEGACY_ENCRYPTED_MEDIA) $(ENABLE_LETTERPRESS) $(ENABLE_MAC_GESTURE_EVENTS) $(ENABLE_MAC_VIDEO_TOOLBOX) $(ENABLE_MATHML) $(ENABLE_MEDIA_CAPTURE) $(ENABLE_MEDIA_CONTROLS_SCRIPT) $(ENABLE_MEDIA_SESSION) $(ENABLE_MEDIA_SOURCE) $(ENABLE_MEDIA_STATISTICS) $(ENABLE_MEDIA_STREAM) $(ENABLE_METER_ELEMENT) $(ENABLE_MHTML) $(ENABLE_MOUSE_CURSOR_SCALE) $(ENABLE_NAVIGATOR_CONTENT_UTILS) $(ENABLE_NAVIGATOR_STANDALONE) $(ENABLE_NOTIFICATIONS) $(ENABLE_PAYMENT_REQUEST) $(ENABLE_PDFKIT_PLUGIN) $(ENABLE_POINTER_EVENTS) $(ENABLE_POINTER_LOCK) $(ENABLE_PUBLIC_SUFFIX_LIST) $(ENABLE_QUOTA) $(ENABLE_REMOTE_INSPECTOR) $(ENABLE_RESOLUTION_MEDIA_QUERY) $(ENABLE_RESOURCE_LOAD_STATISTICS) $(ENABLE_RESOURCE_USAGE) $(ENABLE_RUBBER_BANDING) $(ENABLE_SERVICE_CONTROLS) $(ENABLE_SERVICE_WORKER) $(ENABLE_SPEECH_SYNTHESIS) $(ENABLE_STREAMS_API) $(ENABLE_WEB_CRYPTO) $(ENABLE_SVG_FONTS) $(ENABLE_TELEPHONE_NUMBER_DETECTION) $(ENABLE_TEXT_AUTOSIZING) $(ENABLE_TOUCH_EVENTS) $(ENABLE_USER_MESSAGE_HANDLERS) $(ENABLE_USERSELECT_ALL) $(ENABLE_VARIATION_FONTS) $(ENABLE_VIDEO) $(ENABLE_VIDEO_PRESENTATION_MODE) $(ENABLE_VIDEO_TRACK) $(ENABLE_VIDEO_USES_ELEMENT_FULLSCREEN) $(ENABLE_WEB_API_STATISTICS) $(ENABLE_WEB_AUDIO) $(ENABLE_WEB_AUTHN) $(ENABLE_WEB_RTC) $(ENABLE_WEBASSEMBLY) $(ENABLE_WEBASSEMBLY_STREAMING_API) $(ENABLE_WEBGL) $(ENABLE_WEBGL2) $(ENABLE_WEBGPU) $(ENABLE_WEBMETAL) $(ENABLE_WIRELESS_PLAYBACK_TARGET) $(ENABLE_XSLT);
+FEATURE_DEFINES = $(ENABLE_3D_TRANSFORMS) $(ENABLE_ACCELERATED_2D_CANVAS) $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING) $(ENABLE_ACCESSIBILITY_ISOLATED_TREE) $(ENABLE_APPLE_PAY) $(ENABLE_APPLE_PAY_SESSION_V3) $(ENABLE_APPLE_PAY_SESSION_V4) $(ENABLE_APPLICATION_MANIFEST) $(ENABLE_ATTACHMENT_ELEMENT) $(ENABLE_AVF_CAPTIONS) $(ENABLE_C_LOOP) $(ENABLE_CACHE_PARTITIONING) $(ENABLE_CHANNEL_MESSAGING) $(ENABLE_CONTENT_FILTERING) $(ENABLE_CSS3_TEXT) $(ENABLE_CSS_BOX_DECORATION_BREAK) $(ENABLE_CSS_COMPOSITING) $(ENABLE_CSS_DEVICE_ADAPTATION) $(ENABLE_CSS_IMAGE_ORIENTATION) $(ENABLE_CSS_IMAGE_RESOLUTION) $(ENABLE_CSS_PAINTING_API) $(ENABLE_CSS_SCROLL_SNAP) $(ENABLE_CSS_SELECTORS_LEVEL4) $(ENABLE_CSS_TRAILING_WORD) $(ENABLE_CSS_TYPED_OM) $(ENABLE_CURSOR_VISIBILITY) $(ENABLE_CUSTOM_SCHEME_HANDLER) $(ENABLE_CSS_CONIC_GRADIENTS) $(ENABLE_DARK_MODE_CSS) $(ENABLE_DASHBOARD_SUPPORT) $(ENABLE_DATA_INTERACTION) $(ENABLE_DATACUE_VALUE) $(ENABLE_DATALIST_ELEMENT) $(ENABLE_DEVICE_ORIENTATION) $(ENABLE_DRAG_SUPPORT) $(ENABLE_ENCRYPTED_MEDIA) $(ENABLE_EXPERIMENTAL_FEATURES) $(ENABLE_FAST_JIT_PERMISSIONS) $(ENABLE_FILTERS_LEVEL_2) $(ENABLE_FTL_JIT) $(ENABLE_FULLSCREEN_API) $(ENABLE_GAMEPAD) $(ENABLE_GEOLOCATION) $(ENABLE_ICONDATABASE) $(ENABLE_INDEXED_DATABASE) $(ENABLE_INDEXED_DATABASE_IN_WORKERS) $(ENABLE_INPUT_TYPE_COLOR) $(ENABLE_INPUT_TYPE_DATE) $(ENABLE_INPUT_TYPE_DATETIME_INCOMPLETE) $(ENABLE_INPUT_TYPE_DATETIMELOCAL) $(ENABLE_INPUT_TYPE_MONTH) $(ENABLE_INPUT_TYPE_TIME) $(ENABLE_INPUT_TYPE_WEEK) $(ENABLE_INTERSECTION_OBSERVER) $(ENABLE_INTL) $(ENABLE_IOS_GESTURE_EVENTS) $(ENABLE_IOS_TOUCH_EVENTS) $(ENABLE_JIT) $(ENABLE_KEYBOARD_CODE_ATTRIBUTE) $(ENABLE_KEYBOARD_KEY_ATTRIBUTE) $(ENABLE_LAYOUT_FORMATTING_CONTEXT) $(ENABLE_LEGACY_CSS_VENDOR_PREFIXES) $(ENABLE_LEGACY_CUSTOM_PROTOCOL_MANAGER) $(ENABLE_LEGACY_ENCRYPTED_MEDIA) $(ENABLE_LETTERPRESS) $(ENABLE_MAC_GESTURE_EVENTS) $(ENABLE_MAC_VIDEO_TOOLBOX) $(ENABLE_MATHML) $(ENABLE_MEDIA_CAPTURE) $(ENABLE_MEDIA_CONTROLS_SCRIPT) $(ENABLE_MEDIA_SESSION) $(ENABLE_MEDIA_SOURCE) $(ENABLE_MEDIA_STATISTICS) $(ENABLE_MEDIA_STREAM) $(ENABLE_METER_ELEMENT) $(ENABLE_MHTML) $(ENABLE_MOUSE_CURSOR_SCALE) $(ENABLE_NAVIGATOR_CONTENT_UTILS) $(ENABLE_NAVIGATOR_STANDALONE) $(ENABLE_NOTIFICATIONS) $(ENABLE_PAYMENT_REQUEST) $(ENABLE_PDFKIT_PLUGIN) $(ENABLE_POINTER_EVENTS) $(ENABLE_POINTER_LOCK) $(ENABLE_PUBLIC_SUFFIX_LIST) $(ENABLE_QUOTA) $(ENABLE_REMOTE_INSPECTOR) $(ENABLE_RESOLUTION_MEDIA_QUERY) $(ENABLE_RESOURCE_LOAD_STATISTICS) $(ENABLE_RESOURCE_USAGE) $(ENABLE_RUBBER_BANDING) $(ENABLE_SERVICE_CONTROLS) $(ENABLE_SERVICE_WORKER) $(ENABLE_SPEECH_SYNTHESIS) $(ENABLE_STREAMS_API) $(ENABLE_WEB_CRYPTO) $(ENABLE_SVG_FONTS) $(ENABLE_TELEPHONE_NUMBER_DETECTION) $(ENABLE_TEXT_AUTOSIZING) $(ENABLE_TOUCH_EVENTS) $(ENABLE_USER_MESSAGE_HANDLERS) $(ENABLE_USERSELECT_ALL) $(ENABLE_VARIATION_FONTS) $(ENABLE_VIDEO) $(ENABLE_VIDEO_PRESENTATION_MODE) $(ENABLE_VIDEO_TRACK) $(ENABLE_VIDEO_USES_ELEMENT_FULLSCREEN) $(ENABLE_WEB_API_STATISTICS) $(ENABLE_WEB_AUDIO) $(ENABLE_WEB_AUTHN) $(ENABLE_WEB_RTC) $(ENABLE_WEBASSEMBLY) $(ENABLE_WEBASSEMBLY_STREAMING_API) $(ENABLE_WEBGL) $(ENABLE_WEBGL2) $(ENABLE_WEBGPU) $(ENABLE_WEBMETAL) $(ENABLE_WIRELESS_PLAYBACK_TARGET) $(ENABLE_XSLT);
diff --git a/Source/cmake/WebKitFeatures.cmake b/Source/cmake/WebKitFeatures.cmake
index bca4e99..f70b817 100644
--- a/Source/cmake/WebKitFeatures.cmake
+++ b/Source/cmake/WebKitFeatures.cmake
@@ -193,6 +193,7 @@
     WEBKIT_OPTION_DEFINE(ENABLE_SMOOTH_SCROLLING "Toggle smooth scrolling" PRIVATE ON)
     WEBKIT_OPTION_DEFINE(ENABLE_SPEECH_SYNTHESIS "Toggle Speech Synthesis API support" PRIVATE OFF)
     WEBKIT_OPTION_DEFINE(ENABLE_SPELLCHECK "Toggle Spellchecking support (requires Enchant)" PRIVATE OFF)
+    WEBKIT_OPTION_DEFINE(ENABLE_ACCESSIBILITY_STATIC_TREE "Toggle static accessibility tree support" PRIVATE OFF)
     WEBKIT_OPTION_DEFINE(ENABLE_STREAMS_API "Toggle Streams API support" PRIVATE ON)
     WEBKIT_OPTION_DEFINE(ENABLE_SVG_FONTS "Toggle SVG Fonts support" PRIVATE ON)
     WEBKIT_OPTION_DEFINE(ENABLE_TELEPHONE_NUMBER_DETECTION "Toggle telephone number detection support" PRIVATE OFF)
diff --git a/Tools/ChangeLog b/Tools/ChangeLog
index 1cf82ac..d0b9672 100644
--- a/Tools/ChangeLog
+++ b/Tools/ChangeLog
@@ -1,3 +1,13 @@
+2019-01-27  Chris Fleizach  <cfleizach@apple.com>
+
+        AX: Introduce a static accessibility tree
+        https://bugs.webkit.org/show_bug.cgi?id=193348
+        <rdar://problem/47203295>
+
+        Reviewed by Ryosuke Niwa.
+
+        * TestWebKitAPI/Configurations/FeatureDefines.xcconfig:
+
 2019-01-26  Zalan Bujtas  <zalan@apple.com>
 
         [LFC] The default values for top/bottom in contentHeightForFormattingContextRoot should not be 0.
diff --git a/Tools/TestWebKitAPI/Configurations/FeatureDefines.xcconfig b/Tools/TestWebKitAPI/Configurations/FeatureDefines.xcconfig
index 9122a25..230623e 100644
--- a/Tools/TestWebKitAPI/Configurations/FeatureDefines.xcconfig
+++ b/Tools/TestWebKitAPI/Configurations/FeatureDefines.xcconfig
@@ -40,6 +40,8 @@
 ENABLE_ACCELERATED_OVERFLOW_SCROLLING = $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING_$(WK_COCOA_TOUCH));
 ENABLE_ACCELERATED_OVERFLOW_SCROLLING_cocoatouch = ENABLE_ACCELERATED_OVERFLOW_SCROLLING;
 
+ENABLE_ACCESSIBILITY_ISOLATED_TREE = ;
+
 ENABLE_APPLE_PAY = $(ENABLE_APPLE_PAY_$(WK_PLATFORM_NAME));
 ENABLE_APPLE_PAY_iphoneos = ENABLE_APPLE_PAY;
 ENABLE_APPLE_PAY_iphonesimulator = ENABLE_APPLE_PAY;
@@ -407,4 +409,4 @@
 
 ENABLE_XSLT = ENABLE_XSLT;
 
-FEATURE_DEFINES = $(ENABLE_3D_TRANSFORMS) $(ENABLE_ACCELERATED_2D_CANVAS) $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING) $(ENABLE_APPLE_PAY) $(ENABLE_APPLE_PAY_SESSION_V3) $(ENABLE_APPLE_PAY_SESSION_V4) $(ENABLE_APPLICATION_MANIFEST) $(ENABLE_ATTACHMENT_ELEMENT) $(ENABLE_AVF_CAPTIONS) $(ENABLE_C_LOOP) $(ENABLE_CACHE_PARTITIONING) $(ENABLE_CHANNEL_MESSAGING) $(ENABLE_CONTENT_FILTERING) $(ENABLE_CSS3_TEXT) $(ENABLE_CSS_BOX_DECORATION_BREAK) $(ENABLE_CSS_COMPOSITING) $(ENABLE_CSS_DEVICE_ADAPTATION) $(ENABLE_CSS_IMAGE_ORIENTATION) $(ENABLE_CSS_IMAGE_RESOLUTION) $(ENABLE_CSS_PAINTING_API) $(ENABLE_CSS_SCROLL_SNAP) $(ENABLE_CSS_SELECTORS_LEVEL4) $(ENABLE_CSS_TRAILING_WORD) $(ENABLE_CSS_TYPED_OM) $(ENABLE_CURSOR_VISIBILITY) $(ENABLE_CUSTOM_SCHEME_HANDLER) $(ENABLE_CSS_CONIC_GRADIENTS) $(ENABLE_DARK_MODE_CSS) $(ENABLE_DASHBOARD_SUPPORT) $(ENABLE_DATA_INTERACTION) $(ENABLE_DATACUE_VALUE) $(ENABLE_DATALIST_ELEMENT) $(ENABLE_DEVICE_ORIENTATION) $(ENABLE_DRAG_SUPPORT) $(ENABLE_ENCRYPTED_MEDIA) $(ENABLE_EXPERIMENTAL_FEATURES) $(ENABLE_FAST_JIT_PERMISSIONS) $(ENABLE_FILTERS_LEVEL_2) $(ENABLE_FTL_JIT) $(ENABLE_FULLSCREEN_API) $(ENABLE_GAMEPAD) $(ENABLE_GEOLOCATION) $(ENABLE_ICONDATABASE) $(ENABLE_INDEXED_DATABASE) $(ENABLE_INDEXED_DATABASE_IN_WORKERS) $(ENABLE_INPUT_TYPE_COLOR) $(ENABLE_INPUT_TYPE_DATE) $(ENABLE_INPUT_TYPE_DATETIME_INCOMPLETE) $(ENABLE_INPUT_TYPE_DATETIMELOCAL) $(ENABLE_INPUT_TYPE_MONTH) $(ENABLE_INPUT_TYPE_TIME) $(ENABLE_INPUT_TYPE_WEEK) $(ENABLE_INTERSECTION_OBSERVER) $(ENABLE_INTL) $(ENABLE_IOS_GESTURE_EVENTS) $(ENABLE_IOS_TOUCH_EVENTS) $(ENABLE_JIT) $(ENABLE_KEYBOARD_CODE_ATTRIBUTE) $(ENABLE_KEYBOARD_KEY_ATTRIBUTE) $(ENABLE_LAYOUT_FORMATTING_CONTEXT) $(ENABLE_LEGACY_CSS_VENDOR_PREFIXES) $(ENABLE_LEGACY_CUSTOM_PROTOCOL_MANAGER) $(ENABLE_LEGACY_ENCRYPTED_MEDIA) $(ENABLE_LETTERPRESS) $(ENABLE_MAC_GESTURE_EVENTS) $(ENABLE_MAC_VIDEO_TOOLBOX) $(ENABLE_MATHML) $(ENABLE_MEDIA_CAPTURE) $(ENABLE_MEDIA_CONTROLS_SCRIPT) $(ENABLE_MEDIA_SESSION) $(ENABLE_MEDIA_SOURCE) $(ENABLE_MEDIA_STATISTICS) $(ENABLE_MEDIA_STREAM) $(ENABLE_METER_ELEMENT) $(ENABLE_MHTML) $(ENABLE_MOUSE_CURSOR_SCALE) $(ENABLE_NAVIGATOR_CONTENT_UTILS) $(ENABLE_NAVIGATOR_STANDALONE) $(ENABLE_NOTIFICATIONS) $(ENABLE_PAYMENT_REQUEST) $(ENABLE_PDFKIT_PLUGIN) $(ENABLE_POINTER_EVENTS) $(ENABLE_POINTER_LOCK) $(ENABLE_PUBLIC_SUFFIX_LIST) $(ENABLE_QUOTA) $(ENABLE_REMOTE_INSPECTOR) $(ENABLE_RESOLUTION_MEDIA_QUERY) $(ENABLE_RESOURCE_LOAD_STATISTICS) $(ENABLE_RESOURCE_USAGE) $(ENABLE_RUBBER_BANDING) $(ENABLE_SERVICE_CONTROLS) $(ENABLE_SERVICE_WORKER) $(ENABLE_SPEECH_SYNTHESIS) $(ENABLE_STREAMS_API) $(ENABLE_WEB_CRYPTO) $(ENABLE_SVG_FONTS) $(ENABLE_TELEPHONE_NUMBER_DETECTION) $(ENABLE_TEXT_AUTOSIZING) $(ENABLE_TOUCH_EVENTS) $(ENABLE_USER_MESSAGE_HANDLERS) $(ENABLE_USERSELECT_ALL) $(ENABLE_VARIATION_FONTS) $(ENABLE_VIDEO) $(ENABLE_VIDEO_PRESENTATION_MODE) $(ENABLE_VIDEO_TRACK) $(ENABLE_VIDEO_USES_ELEMENT_FULLSCREEN) $(ENABLE_WEB_API_STATISTICS) $(ENABLE_WEB_AUDIO) $(ENABLE_WEB_AUTHN) $(ENABLE_WEB_RTC) $(ENABLE_WEBASSEMBLY) $(ENABLE_WEBASSEMBLY_STREAMING_API) $(ENABLE_WEBGL) $(ENABLE_WEBGL2) $(ENABLE_WEBGPU) $(ENABLE_WEBMETAL) $(ENABLE_WIRELESS_PLAYBACK_TARGET) $(ENABLE_XSLT);
+FEATURE_DEFINES = $(ENABLE_3D_TRANSFORMS) $(ENABLE_ACCELERATED_2D_CANVAS) $(ENABLE_ACCELERATED_OVERFLOW_SCROLLING) $(ENABLE_ACCESSIBILITY_ISOLATED_TREE) $(ENABLE_APPLE_PAY) $(ENABLE_APPLE_PAY_SESSION_V3) $(ENABLE_APPLE_PAY_SESSION_V4) $(ENABLE_APPLICATION_MANIFEST) $(ENABLE_ATTACHMENT_ELEMENT) $(ENABLE_AVF_CAPTIONS) $(ENABLE_C_LOOP) $(ENABLE_CACHE_PARTITIONING) $(ENABLE_CHANNEL_MESSAGING) $(ENABLE_CONTENT_FILTERING) $(ENABLE_CSS3_TEXT) $(ENABLE_CSS_BOX_DECORATION_BREAK) $(ENABLE_CSS_COMPOSITING) $(ENABLE_CSS_DEVICE_ADAPTATION) $(ENABLE_CSS_IMAGE_ORIENTATION) $(ENABLE_CSS_IMAGE_RESOLUTION) $(ENABLE_CSS_PAINTING_API) $(ENABLE_CSS_SCROLL_SNAP) $(ENABLE_CSS_SELECTORS_LEVEL4) $(ENABLE_CSS_TRAILING_WORD) $(ENABLE_CSS_TYPED_OM) $(ENABLE_CURSOR_VISIBILITY) $(ENABLE_CUSTOM_SCHEME_HANDLER) $(ENABLE_CSS_CONIC_GRADIENTS) $(ENABLE_DARK_MODE_CSS) $(ENABLE_DASHBOARD_SUPPORT) $(ENABLE_DATA_INTERACTION) $(ENABLE_DATACUE_VALUE) $(ENABLE_DATALIST_ELEMENT) $(ENABLE_DEVICE_ORIENTATION) $(ENABLE_DRAG_SUPPORT) $(ENABLE_ENCRYPTED_MEDIA) $(ENABLE_EXPERIMENTAL_FEATURES) $(ENABLE_FAST_JIT_PERMISSIONS) $(ENABLE_FILTERS_LEVEL_2) $(ENABLE_FTL_JIT) $(ENABLE_FULLSCREEN_API) $(ENABLE_GAMEPAD) $(ENABLE_GEOLOCATION) $(ENABLE_ICONDATABASE) $(ENABLE_INDEXED_DATABASE) $(ENABLE_INDEXED_DATABASE_IN_WORKERS) $(ENABLE_INPUT_TYPE_COLOR) $(ENABLE_INPUT_TYPE_DATE) $(ENABLE_INPUT_TYPE_DATETIME_INCOMPLETE) $(ENABLE_INPUT_TYPE_DATETIMELOCAL) $(ENABLE_INPUT_TYPE_MONTH) $(ENABLE_INPUT_TYPE_TIME) $(ENABLE_INPUT_TYPE_WEEK) $(ENABLE_INTERSECTION_OBSERVER) $(ENABLE_INTL) $(ENABLE_IOS_GESTURE_EVENTS) $(ENABLE_IOS_TOUCH_EVENTS) $(ENABLE_JIT) $(ENABLE_KEYBOARD_CODE_ATTRIBUTE) $(ENABLE_KEYBOARD_KEY_ATTRIBUTE) $(ENABLE_LAYOUT_FORMATTING_CONTEXT) $(ENABLE_LEGACY_CSS_VENDOR_PREFIXES) $(ENABLE_LEGACY_CUSTOM_PROTOCOL_MANAGER) $(ENABLE_LEGACY_ENCRYPTED_MEDIA) $(ENABLE_LETTERPRESS) $(ENABLE_MAC_GESTURE_EVENTS) $(ENABLE_MAC_VIDEO_TOOLBOX) $(ENABLE_MATHML) $(ENABLE_MEDIA_CAPTURE) $(ENABLE_MEDIA_CONTROLS_SCRIPT) $(ENABLE_MEDIA_SESSION) $(ENABLE_MEDIA_SOURCE) $(ENABLE_MEDIA_STATISTICS) $(ENABLE_MEDIA_STREAM) $(ENABLE_METER_ELEMENT) $(ENABLE_MHTML) $(ENABLE_MOUSE_CURSOR_SCALE) $(ENABLE_NAVIGATOR_CONTENT_UTILS) $(ENABLE_NAVIGATOR_STANDALONE) $(ENABLE_NOTIFICATIONS) $(ENABLE_PAYMENT_REQUEST) $(ENABLE_PDFKIT_PLUGIN) $(ENABLE_POINTER_EVENTS) $(ENABLE_POINTER_LOCK) $(ENABLE_PUBLIC_SUFFIX_LIST) $(ENABLE_QUOTA) $(ENABLE_REMOTE_INSPECTOR) $(ENABLE_RESOLUTION_MEDIA_QUERY) $(ENABLE_RESOURCE_LOAD_STATISTICS) $(ENABLE_RESOURCE_USAGE) $(ENABLE_RUBBER_BANDING) $(ENABLE_SERVICE_CONTROLS) $(ENABLE_SERVICE_WORKER) $(ENABLE_SPEECH_SYNTHESIS) $(ENABLE_STREAMS_API) $(ENABLE_WEB_CRYPTO) $(ENABLE_SVG_FONTS) $(ENABLE_TELEPHONE_NUMBER_DETECTION) $(ENABLE_TEXT_AUTOSIZING) $(ENABLE_TOUCH_EVENTS) $(ENABLE_USER_MESSAGE_HANDLERS) $(ENABLE_USERSELECT_ALL) $(ENABLE_VARIATION_FONTS) $(ENABLE_VIDEO) $(ENABLE_VIDEO_PRESENTATION_MODE) $(ENABLE_VIDEO_TRACK) $(ENABLE_VIDEO_USES_ELEMENT_FULLSCREEN) $(ENABLE_WEB_API_STATISTICS) $(ENABLE_WEB_AUDIO) $(ENABLE_WEB_AUTHN) $(ENABLE_WEB_RTC) $(ENABLE_WEBASSEMBLY) $(ENABLE_WEBASSEMBLY_STREAMING_API) $(ENABLE_WEBGL) $(ENABLE_WEBGL2) $(ENABLE_WEBGPU) $(ENABLE_WEBMETAL) $(ENABLE_WIRELESS_PLAYBACK_TARGET) $(ENABLE_XSLT);