Intent to prototype and ship: Make <link> elements not traversable

[Emilio Cobos Álvarez]

(I forgot to send an intent for this a couple weeks ago)

Summary: This is a bit of a mess, see [1][2]. TLDR:

* Gecko treated <link> elements like anchors, in the sense that if you
display them, we allowed you to click around, etc. This meant also
supporting :link / :visited on them, and such. We stopped supporting
:visited on them a while back[3], because doing history queries for that
and expose more attack surface for timing attacks seems just bad.

* WebKit matched :link / :visited but didn't allow you to navigate
using them. They stopped matching :visited around the same time as we
did, too.

* Blink at some point long time ago stopped matching :link / :visited
on <link> elements.

This change aligns us with Blink (and the updated HTML spec).

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1687538

DevTools bug: N/A, I think

Other browsers: see above

Standard: https://github.com/whatwg/html/pull/6269

web-platform-tests: Landed with the html spec change.

[1]: https://github.com/w3c/csswg-drafts/issues/3817
[2]: https://github.com/whatwg/html/issues/4831
[3]:
https://groups.google.com/d/msg/mozilla.dev.platform/1NP6oJzK6zg/uUq9bFLaAAAJ