Use xref, avoid data-cite="html"

[marcoscaceres]

The spec uses data-cite="!HTML/#thing" in various places. With "xref", you can just link directly to things in the HTML spec (and other specs that are part of browser specs).

For xref usage, please see:
https://respec.org/docs/#xref

[dontcallmedom]

This is being used for the following anchors at the moment:

• dom-media-have_metadata
• ended-playback
• gains-focus
• media-timeline
• potentially-playing

Except for dom-media-have_metadata, these are all marked as private definitions at the moment, so not usable through xref which is why we've fallen back on doing it this way.

I'm not sure why dom-media-have_metadata doesn't show up in xref.

Getting these fixed upstream has been in my todo list, but admittedly not very high so far :)

I did file #795 to address a case that had no justification for not using xref.

[dontcallmedom]

for gain-focus, the real problem is that we're not targeting the proper definition in any case, which is tied to the discussion of #752

[jan-ivar]

@dontcallmedom Note #752 has since merged in case it unblocks this.