HTMLLinkElement load emit timing

[avaer]

This case was not properly triggering load events:

const link = document.createElement('link');
link.href = 'https://fonts.googleapis.com/css?family=Source+Code+Pro';
link.rel = 'stylesheet';
link.onload = () => {
  console.log('got load');
];

This was mainly because of the "run html" flow for HTMLLinkElement, which handled things synchronously on attribute set rather than working on the defer-emits-to-document-load model like HTMLScriptElement.

This PR makes HTMLLinkElement behave like HTMLScriptElement in that regard.

This was needed to boot cryptovoxels.