LayoutTests/fast/css/cssom-mutation-stylerule.html

<style>
    aaa {
        color: red;
    }
</style>
<script>
  onload = () => {
    let cssStyleRule = document.styleSheets[0].cssRules[0];
    document.styleSheets[0].deleteRule(0);
    cssStyleRule.insertRule(`.bar { color: black; }`);
  };
</script>

Test passes if no crash.