<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" onload="doTest()">
  <style>
    .background { fill: lime }
    .workspace  { fill: red  }
    .test       { fill: lime }
  </style>
  <defs>
    <!-- 'Dummy' path -->
    <path id="moveFarAway"      d="M300,300 h0"/>

    <path id="moveToUpperLeft"  d="M100,100 h0"/>

    <path id="pathWhoseDAttrChanges" d="M360,360 h0"/>

    <!-- The first of these two elems w/ same ID should be used. -->
    <path id="moveToMiddleLeft"  d="M100,150 h0"/>
    <path id="moveToMiddleLeft"  d="M350,350 h0"/>

    <!-- The first of these two elems w/ same ID initially wins, but then
         it gets removed via script. -->
    <path id="moveToMiddleCenter" d="M340,340 h0"/>
    <path id="moveToMiddleCenter" d="M150,150 h0"/>

    <!-- This elem doesn't do what its id would suggest, but we'll use JS to
         add an earlier elem with the same ID that *does* do what it says. -->
    <path id="moveToMiddleRight" d="M330,330 h0"/>

    <path id="moveToLowerLeft"   d="M100,200 h0"/>
    <path id="moveToLowerCenter" d="M150,200 h0"/>

  </defs>

  <script type="text/javascript">
    const SVGNS   = "http://www.w3.org/2000/svg";
    const XLINKNS = "http://www.w3.org/1999/xlink";

    if (window.testRunner)
        testRunner.waitUntilDone();

    function insertPathElem(aPathId, aPathSpec) {
      var newPath = document.createElementNS(SVGNS, "path");
      newPath.setAttribute("id", aPathId);
      newPath.setAttribute("d",  aPathSpec);

      // Insert new path into defs
      var defsElem = document.getElementsByTagName("defs")[0];
      defsElem.insertBefore(newPath, defsElem.firstChild);
    }

    function doTest() {
      // Seek already, so we'll have sampled the initial 'stale' state
      document.documentElement.setCurrentTime(1);

      // Make tweaks
      var mpathToModify = document.getElementById("modifyMyTarget");
      mpathToModify.setAttributeNS(XLINKNS, "href", "#moveToUpperLeft");

      var mpathWhoseHrefNeedsClearing = document.getElementById("unsetMyTarget");
      mpathWhoseHrefNeedsClearing.removeAttributeNS(XLINKNS, "href");

      var pathToTweak = document.getElementById("pathWhoseDAttrChanges");
      pathToTweak.setAttribute("d", "M200 100 h0");

      var mpathToDelete = document.getElementById("removeMe");
      mpathToDelete.parentNode.removeChild(mpathToDelete);

      var pathToDelete = document.getElementById("moveToMiddleCenter");
      pathToDelete.parentNode.removeChild(pathToDelete);

      insertPathElem("moveToMiddleRight", "M200,150 h0");
      insertPathElem("moveToLowerRight", "M200,200 h0");

      setTimeout(function() {
        document.documentElement.setCurrentTime(1);
        document.documentElement.pauseAnimations();
        if (window.testRunner)
          testRunner.notifyDone();
      }, 0);
    }
  </script>

  <!-- Big green background to match lime.svg -->
  <rect class="background" width="100%" height="100%" />
  <!-- Red "workspace" (should be covered up, if tests pass) -->
  <rect class="workspace" x="100" y="100" width="150" height="150"/>

  <!-- FIRST ROW: Test behavior... -->
  <!-- ...when mpath's 'xlink:href' attr is modified. -->
  <rect class="test" x="0" y="0" width="50" height="50">
    <animateMotion begin="1" dur="1" fill="freeze">
      <mpath id="modifyMyTarget" xlink:href="#moveFarAway"/>
    </animateMotion>
  </rect>

  <!-- ...when mpath's 'xlink:href' is unset.  -->
  <rect class="test" x="150" y="100" width="50" height="50">
    <animateMotion begin="1" dur="1" fill="freeze">
      <mpath id="unsetMyTarget" xlink:href="#moveFarAway"/>
    </animateMotion>
  </rect>

  <!-- ...when the target-path's "d" attr is modified. -->
  <rect class="test" x="0" y="0" width="50" height="50">
    <animateMotion begin="1" dur="1" fill="freeze">
      <mpath xlink:href="#pathWhoseDAttrChanges"/>
    </animateMotion>
  </rect>

  <!-- SECOND ROW: Test behavior... -->
  <!--  ...when there are two paths with same ID (first should win) -->
  <rect class="test" x="0" y="0" width="50" height="50">
    <animateMotion begin="1" dur="1" fill="freeze">
      <mpath xlink:href="#moveToMiddleLeft"/>
    </animateMotion>
  </rect>

  <!--  ...when there are two paths with same ID, and the first is removed. -->
  <rect class="test" x="0" y="0" width="50" height="50">
    <animateMotion begin="1" dur="1" fill="freeze">
      <mpath xlink:href="#moveToMiddleCenter"/>
    </animateMotion>
  </rect>

  <!--  ...when an earlier path is added with our target ID. -->
  <rect class="test" x="0" y="0" width="50" height="50">
    <animateMotion begin="1" dur="1" fill="freeze">
      <mpath xlink:href="#moveToMiddleRight"/>
    </animateMotion>
  </rect>

  <!-- THIRD ROW: Test behavior... -->
  <!--  ...when there are two mpath children (first should win). -->
  <rect class="test" x="0" y="0" width="50" height="50">
    <animateMotion begin="1" dur="1" fill="freeze">
      <mpath xlink:href="#moveToLowerLeft"/>
      <mpath xlink:href="#moveFarAway"/>
    </animateMotion>
  </rect>

  <!--  ...when there are two mpath children, and the first is removed. -->
  <rect class="test" x="0" y="0" width="50" height="50">
    <animateMotion begin="1" dur="1" fill="freeze">
      <mpath id="removeMe" xlink:href="#moveFarAway"/>
      <mpath xlink:href="#moveToLowerCenter"/>
    </animateMotion>
  </rect>
  <!--  ...when there's an mpath child that initially matches nothing, until
        a node with the right ID is inserted into the DOM. -->
  <rect class="test" x="0" y="0" width="50" height="50">
    <animateMotion begin="1" dur="1" fill="freeze">
      <mpath xlink:href="#moveToLowerRight"/>
    </animateMotion>
  </rect>
</svg>