Currently, implementations are required to fail this test

test(t => {
  const vid = document.createElement("video");
  t.add_cleanup(() => vid.remove());
  vid.setAttribute("muted", "");
  document.body.append(vid);
  assert_true(vid.muted);
}, "<video muted> in the light tree");

but Safari passes it. And that's because the muted content attribute is supposed to be checked during the creation of the element, which is an anti-pattern that we explicitly decided not to enable for custom elements and would also try to get rid of where it appears, I think.

Ideally we'd evaluate the muted content attribute upon insertion or some such, rather than during element creation.

cc @whatwg/media