mozilla-release/layout/reftests/stylesheet-cloning/supports-rule-clone.html

<!DOCTYPE html>
<link rel="stylesheet" href="data:text/css,@supports (color: green) {div { color: green; }}">
<link rel="stylesheet" href="data:text/css,@supports (color: green) {div { color: green; }}">
<div>This should be green</div>
<script>
  onload = function() {
    var links = document.getElementsByTagName("link");
    links[0].sheet.cssRules[0].cssRules[0].style.color = "red";
  }
</script>