third_party/blink/web_tests/svg/custom/percent-height.svg

<svg xmlns="http://www.w3.org/2000/svg" onload="startTest()">
<script>
if (self.internals)
  internals.runtimeFlags.overlayScrollbarsEnabled = true;

function startTest() {
  if (!window.testRunner)
    return;

  testRunner.dumpAsText();
  testRunner.waitUntilDone();
  testRunner.setBackingScaleFactor(2, finishTest);
  var rect = document.getElementById('rect');
  var result = document.getElementById('result');
  result.innerHTML = 'rect.offsetHeight = ' + rect.getBoundingClientRect().height;
}

function finishTest() {
  setTimeout(function () { testRunner.notifyDone(); }, 0);
}
</script>

<mask id='mask'>
  <rect id='rect' width='100%' height='100%' fill='white'/>
</mask>
<text id='result'></text>
</svg>