diff --git a/Source/WebCore/ChangeLog b/Source/WebCore/ChangeLog
index 842269b..2eb346b 100644
--- a/Source/WebCore/ChangeLog
+++ b/Source/WebCore/ChangeLog
@@ -1,3 +1,25 @@
+2020-06-30  Sergio Villar Senin  <svillar@igalia.com>
+
+        [css-flex] Remove death code paths when evaluating percentage resolution
+        https://bugs.webkit.org/show_bug.cgi?id=213809
+
+        Reviewed by Manuel Rego Casasnovas.
+
+        Both crossSizeForPercentageResolution() and mainSizeForPercentageResolution() are only called from
+        childLogicalHeightForPercentageResolution(). The former is called whenever hasOrthogonalFlow(child)
+        is false and the latter when it's true. However crossSizeForPercentageResolution() has a path for
+        hasOrthogonalFlow(child)==true which is impossible to reach. The same happens to
+        mainSizeForPercentageResolution() which has a path for hasOrthogonalFlow(child)==false which is
+        also impossible to reach.
+
+        Remove both death code paths and replace them by assertions. We're also making both methods
+        private since are not meant to be used from the outside.
+
+        * rendering/RenderFlexibleBox.cpp:
+        (WebCore::RenderFlexibleBox::crossSizeForPercentageResolution): Remove death code path.
+        (WebCore::RenderFlexibleBox::mainSizeForPercentageResolution): Ditto.
+        * rendering/RenderFlexibleBox.h: Make both calls private.
+
 2020-06-30  Sam Weinig  <weinig@apple.com>
 
         Split Color serialization out of Color classes
diff --git a/Source/WebCore/rendering/RenderFlexibleBox.cpp b/Source/WebCore/rendering/RenderFlexibleBox.cpp
index acafdfd..2547b75 100644
--- a/Source/WebCore/rendering/RenderFlexibleBox.cpp
+++ b/Source/WebCore/rendering/RenderFlexibleBox.cpp
@@ -1161,13 +1161,12 @@
     
 Optional<LayoutUnit> RenderFlexibleBox::crossSizeForPercentageResolution(const RenderBox& child)
 {
+    ASSERT(!hasOrthogonalFlow(child));
     if (alignmentForChild(child) != ItemPosition::Stretch)
         return WTF::nullopt;
 
     // Here we implement https://drafts.csswg.org/css-flexbox/#algo-stretch
-    if (hasOrthogonalFlow(child) && child.hasOverrideContentLogicalWidth())
-        return child.overrideContentLogicalWidth();
-    if (!hasOrthogonalFlow(child) && child.hasOverrideContentLogicalHeight())
+    if (child.hasOverrideContentLogicalHeight())
         return child.overrideContentLogicalHeight();
     
     // We don't currently implement the optimization from
@@ -1181,15 +1180,14 @@
 
 Optional<LayoutUnit> RenderFlexibleBox::mainSizeForPercentageResolution(const RenderBox& child)
 {
+    ASSERT(hasOrthogonalFlow(child));
     // This function implements section 9.8. Definite and Indefinite Sizes, case 2) of the flexbox spec.
     // If the flex container has a definite main size the flex item post-flexing main size is also treated
     // as definite. We make up a percentage to check whether we have a definite size.
     if (!mainAxisLengthIsDefinite(child, Length(0, Percent)))
         return WTF::nullopt;
 
-    if (hasOrthogonalFlow(child))
-        return child.hasOverrideContentLogicalHeight() ? Optional<LayoutUnit>(child.overrideContentLogicalHeight()) : WTF::nullopt;
-    return child.hasOverrideContentLogicalWidth() ? Optional<LayoutUnit>(child.overrideContentLogicalWidth()) : WTF::nullopt;
+    return child.hasOverrideContentLogicalHeight() ? Optional<LayoutUnit>(child.overrideContentLogicalHeight()) : WTF::nullopt;
 }
 
 Optional<LayoutUnit> RenderFlexibleBox::childLogicalHeightForPercentageResolution(const RenderBox& child)
diff --git a/Source/WebCore/rendering/RenderFlexibleBox.h b/Source/WebCore/rendering/RenderFlexibleBox.h
index 15afbb7..c4b0373 100644
--- a/Source/WebCore/rendering/RenderFlexibleBox.h
+++ b/Source/WebCore/rendering/RenderFlexibleBox.h
@@ -69,8 +69,6 @@
 
     virtual bool isFlexibleBoxImpl() const { return false; };
     
-    Optional<LayoutUnit> crossSizeForPercentageResolution(const RenderBox&);
-    Optional<LayoutUnit> mainSizeForPercentageResolution(const RenderBox&);
     Optional<LayoutUnit> childLogicalHeightForPercentageResolution(const RenderBox&);
     
     void clearCachedMainSizeForChild(const RenderBox& child);
@@ -155,7 +153,9 @@
     Overflow mainAxisOverflowForChild(const RenderBox& child) const;
     Overflow crossAxisOverflowForChild(const RenderBox& child) const;
     void cacheChildMainSize(const RenderBox& child);
-    
+    Optional<LayoutUnit> crossSizeForPercentageResolution(const RenderBox&);
+    Optional<LayoutUnit> mainSizeForPercentageResolution(const RenderBox&);
+
     void layoutFlexItems(bool relayoutChildren);
     LayoutUnit autoMarginOffsetInMainAxis(const Vector<FlexItem>&, LayoutUnit& availableFreeSpace);
     void updateAutoMarginsInMainAxis(RenderBox& child, LayoutUnit autoMarginOffset);