css/cssom/HTMLLinkElement-load-event.html

<!doctype html>
<meta charset="utf-8">
<title>Link element load event doesn't block the parser.</title>
<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
<link rel="author" title="Mozilla" href="https://mozilla.org">
<link rel="help" href="https://html.spec.whatwg.org/#link-type-stylesheet">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script>
  let NUM_LOADS = 0;
</script>
<link rel="stylesheet" href="data:text/css,*{}" onload="++NUM_LOADS">
<script>
let t = async_test(document.title);
window.addEventListener("load", t.step_func_done(() => {
  assert_equals(NUM_LOADS, 2, "Load event should've fired for all links");
}));
t.step(function() {
  assert_equals(document.styleSheets.length, 1, "Should expose the sheet to the OM before running script");
  // We can't quite assert that NUM_LOADS is zero (even though it almost-always
  // should be the case), in case the parser yields just before executing the
  // script but after parsing the link load.
  let loadsBefore = NUM_LOADS;
  // Intentionally the same href as above, to test caching behavior.
  document.write(`
    <link rel="stylesheet" href="data:text/css,*{}" onload="++NUM_LOADS">
  `);
  assert_equals(document.styleSheets.length, 2, "Should expose both sheets to the OM before running second script");
  assert_equals(NUM_LOADS, loadsBefore, "Shouldn't fire the load event sync");
  assert_not_equals(document.styleSheets[0], document.styleSheets[1], "Should be different sheets");
});
</script>