Add <a> to spec

[NSoiffer]

As agreed to in the Dec 4, 2025 meeting, <a> should be added to the spec. It should be considered to be identical to <mrow> with the comment that in core, the href attr works.

[davidcarlisle]

This has been added by PR #558 however the added example prevents the publishing to TR space so currently it is only in github pages.

  💥 Echidna publish has failed.
  Specberus Errors:
    [6177@171] Element “a” not allowed as child of element “math” in this context. (Suppressing further errors from this subtree.)<br><a href="https://validator.w3.org/nu/?doc=https://mimas.w3.org/6f0defaf-de76-4562-bb78-26613c8f4499/Overview.html">HTML validator</a>
      { name: 'validation.html', section: 'format', rule: 'validHTML' }

@NSoiffer I could mark the example as not core, then the build would not inline the example, which might be an appropriate quick fix to get the update to TR space

[davidcarlisle]

Ok finally it passes echidna validation, basically the example displays as <a href but uses mrow and onclick events in the inlined display.