transition-3d-transform-will-change-contents.html

<!DOCTYPE html>
<style>
#target {
  width:100px;
  height: 100px;
  background: blue;
  transition-duration: 0.1s;
  transform: translate3d(10px, 0, 0);
  will-change: contents;
}
#target.changed {
  width: 400px;
  transform: translate3d(20px, 0, 0);
}
</style>
<div id="target"></div>
<script src="../resources/run-after-layout-and-paint.js"></script>
<script>
if (window.testRunner)
  testRunner.waitUntilDone();
runAfterLayoutAndPaint(() => {
  target.addEventListener('transitionend', () => {
    if (window.testRunner)
     testRunner.notifyDone();
  });
  target.classList.add('changed');
});
</script>

chromium/third_party/blink/web_tests/transitions/transition-3d-transform-will-change-contents.html