Proposal: Make style ids immutable

[Dan_Stillman]

If we wanted to support non-repo parents, <parent-id> would be cleaner, since it would allow all <link>s to remain URLs, which would be more consistent.

I think it’d only make sense to specify rel="independent-parent" as an id if we didn’t actually care about supporting non-repo parents and just wanted to fix the spec.

Also, as I say above:

@Dan_Stillman

If we did [add <parent-id>], I think we’d have to say that an independent-parent in the absence of a <parent-id> was also an id so that existing styles without <parent-id> continued to work.