css/css-writing-modes/dynamic-offset-rtl-001.html

<!DOCTYPE HTML>
<head>
<meta charset="UTF-8">
<title>CSS Test: dynamic changes to offset properties</title>
<link rel="author" title="L. David Baron" href="https://dbaron.org/">
<link rel="author" title="Mozilla" href="https://www.mozilla.org/">
<link rel="help" href="https://drafts.csswg.org/css-writing-modes-3/#vertical-layout">
<link rel="match" href="dynamic-offset-rtl-001-ref.html">
<meta name="flags" content="dom">
<meta name="assert" content="Layout rules for 'right' and 'left' are handled correctly for absolutely positioned elements, in the presence of dynamic changes to 'width' of the containing block.">
<style>

html {
  direction: rtl;
  overflow: hidden;
}

#container {
  position: absolute;
  top: 0;
  left: 0;
  width: 200px;
  height: 100px;
  background: yellow;
}

#abspos {
  position: absolute;
  top: 10px;
  right: 20px;
  width: 10px;
  height: 10px;
  background: fuchsia;
}

</style>

<div id="container">
  <div id="abspos"></div>
</div>

<script>

window.addEventListener("load", function(event) {
  var e = document.getElementById("abspos");
  e.offsetTop; // flush layout
  e.parentNode.style.width = "100px";
});

</script>