LayoutTests/fast/frames/stylesheet-mutation-focus-crash.html

<style>
  #z, :is(:focus-visible) {
    width: 0;
  }
</style>
<script>
  if (window.testRunner)
    testRunner.dumpAsText();

  document.designMode = "on";

  onload = () => {
    i0.onblur = () => {
      document.styleSheets[0].cssRules[0].selectorText = `#w`;
    };
    i0.focus();
    document.body.offsetLeft;
    i1.focus();
  };
</script>
<iframe id="i0"></iframe>
<iframe id="i1"></iframe>
This test passes if it doesn't crash.