css/css-variables/variable-generated-content-dynamic-001.html

<!DOCTYPE html>
<meta charset="utf-8">
<title>CSS Test: Dynamic attribute change handling in generated content</title>
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
<link rel="help" href="https://drafts.csswg.org/css-variables/#using-variables">
<link rel="match" href="variable-generated-content-dynamic-001-ref.html">
<style>
div::before {
  --my-attr: attr(data-foo);
  content: var(--my-attr);
}
</style>
<div data-foo="FAIL"></div>
<script>
document.body.offsetTop;
document.querySelector('div').setAttribute('data-foo', "PASS");
</script>