LayoutTests/fast/css/delete-namespace-rule-when-child-rule-exists.html

<style>
  @namespace a url();
</style>
<script src="../../resources/js-test.js"></script>
<script>
  description("Tests that deleting a @namespace rule when list contains anything other than @import or @namespace rules should throw InvalidStateError.");

  let styleSheet = document.styleSheets[0];
  styleSheet.cssRules[0];
  styleSheet.insertRule(`b {}`, 1);
  shouldThrowErrorName("styleSheet.deleteRule(0)", "InvalidStateError");
</script>