Fix broken refs to HTML spec, adjust settings for FPWD

[tidoust]

The spec referenced a number of terms in the HTML spec using links to the superseded HTML 5.1 spec. That spec actually redirects to the WHATWG HTML LS spec, which is good. Problem is a good chunk of anchors have been modified in the HTML spec and are no longer valid.

This update fixes all broken fragments using a combination of:

• "xref" terms, automatically handled by ReSpec, when possible (i.e. when the HTML spec exports them).
• fragment updates in media-source.js when the spec needs to reference non-exported terms. Longer-term, we'll need to work with HTML editors to make sure that the spec properly exports the terms that we need to reference.

These updates are notably needed to pass pubrules for publication as First Public Working Draft (I included them in the version I sent for publication to /TR).

One note on xref terms: the HTML spec no longer has the concept of "initial playback position". The concept seems to have been merged with "current playback position".

This update also includes a couple of other fixes to please pubrules:

• Required editor ID was missing for Mark.
• Link to Can I Use was semi-broken (they still managed to redirect the request but a more direct https://caniuse.com/mediasource can be used)

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[tidoust]

I added a couple of updates to use a leveled shortname because it turns out that it is required and to drop the link to the now outdated implementation report.