PSA: Dynamically inserted StyleSheet will not block loading.

After [1] landed, dynamically inserted StyleSheet will not block loading.

Before this patch, Chrome would stop parsing HTML when downloading programmatically injected CSS.

But according to the HTML5 spec [2], the injected CSS should not be blocking.

Example:

  <script>

  var link = document.createElement('link');

  link.setAttribute('rel', 'stylesheet');

  link.setAttribute('type', 'text/css');

  // This CSS returns "div { color: red; }" with a 10 seconds delay

  link.setAttribute('href', 'really-really-slow.css');

  document.head.appendChild(link);

  </script>

  <div>This should be black then red.</div>

Before [1]:

  The text in the div doesn't show up initially. After about 10 seconds (when really-really-slow.css is loaded), it shows up and it's red from the beginning.

After [1]:

  The text in the div is black initially. After about 10 seconds (when really-really-slow.css is loaded), it turns red.

[1] https://codereview.chromium.org/1603873004

[2] https://html.spec.whatwg.org/multipage/semantics.html#a-style-sheet-that-is-blocking-scripts

For more, see http://crbug.com/571725.