<!DOCTYPE html>
<style>
#hitregion {
  width: 100px;
  height: 100px;
  background-color: green;
}
#hitregion:after {
  content: "";
  border: 50px solid;
  position: absolute;
  width: 0;
  height: 0;
  margin-left: 140px;
}

#hitregion:hover:after {
  color: green;
}
</style>
<div id="hitregion"></div>
<script>
if (window.eventSender) {
  testRunner.dumpAsText();

  window.internals.startTrackingRepaints();

  eventSender.mouseMoveTo(50, 50);
  eventSender.mouseDown();
  eventSender.mouseUp();

  document.body.offsetHeight;

  let repaintRects = window.internals.repaintRectsAsText();
  window.internals.stopTrackingRepaints();

  hitregion.innerText = repaintRects;
}
</script>