Media element resource selection algorithm: resolve the URL lazily, matching implementations

https://html.spec.whatwg.org/multipage/embedded-content.html#concept-media-load-algorithm

The problem exists in two places:

"Let urlString be the resulting URL string that would have resulted from parsing the URL specified by the src attribute's value relative to the media element's node document when the src attribute was last changed."

"Let urlString be the resulting URL string that would have resulted from parsing the URL specified by candidate's src attribute's value relative to the candidate's node document when the src attribute was last changed."

I never implemented this in Presto, and it's not what happens in WebKit or Blink either. Instead the URL is just resolved right then and there. There's a difference when the document's base URL changes in between the two times, but it doesn't seem to matter in practice.

@padenot, what does Gecko do?

@DigiTec, who's a good contact for media element issues in Edge?

Start with the media element resource selection algorithm at the linked HTML Standard section. Investigate how Gecko, WebKit, Blink, and Edge resolve media URLs when the document base URL changes, then determine whether the specification should match implementations or retain lazy resolution; done means a documented decision and corresponding specification update.