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chromium/third_party/blink/web_tests/external/wpt/layout-instability/sources.html 

<!DOCTYPE html>
<title>Layout Instability: sources attribute</title>
<link rel="help" href="https://wicg.github.io/layout-instability/" />
<style>

body { margin: 10px; }
#shifter { position: relative; width: 300px; height: 100px; background: blue; }

</style>
<div id="shifter"></div>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="resources/util.js"></script>
<script>

strrect = r => `[${r.x},${r.y},${r.width},${r.height}]`;

promise_test(async () => {
  const watcher = new ScoreWatcher;
  const shifter = document.querySelector("#shifter");

  // Wait for the initial render to complete.
  await waitForAnimationFrames(2);

  // Modify the position of the div.
  shifter.style = "top: 60px; left: 10px";
  await watcher.promise;

  const sources = watcher.lastEntry.sources;
  assert_equals(sources.length, 1);

  const source = sources[0];
  assert_equals(source.node, shifter);
  assert_equals(strrect(source.previousRect), "[10,10,300,100]");
  assert_equals(strrect(source.currentRect), "[20,70,300,100]");
}, "Sources attribute.");

</script>