Prohibit relative links in HTML document (except for "additional elements" and for "external code snippets")

[thomasheritage]

Context

If an author includes something like the following in the HTML document they are writing:

<a href="../registers/">registers directory</a>

then this will appear as a relative link in the HTML rendering.

This works on the author's machine because it links to other resources in the same repo as the HTML document.

This will only work in the HTML rendering on https://doc.smpte-doc.org/ if the referenced item has been included in what's automatically copied to the S3 bucket.

Such links are converted to absolute links in the generated PDF file. For example:

file:///home/runner/work/st2067-206-private/st2067-206-private/build/registers/

which is clearly not helpful.

Proposal

Inclusion of relative links in the HTML document is a validation error except that:

• they can be used in the "Additional elements" Clause (presumably additional elements files must not be linked to directly from elsewhere in the document though)
• they can be used for "external code snippets"
• they can be used for images
• of course fragment-only links are allowed...

Would that make sense?