Allow <a rel="me"> to pass the sanitiser?

[GreenReaper]

While trying to verify a link from a Mastodon instance to a Mailman Postorius mailing list profile, I found that the rel="me" attribute on the reciprocal link back to Mastodon had been replaced by a rel="nofollow". After a little digging I found they were using this module, which does not include rel in the list of allowed attributes for <a>.

Would it be possible to enable <a rel>, or at least <a rel="me">? There are some problematic uses of rel, such as canonical, but they don't seem to be allowed in the variant of the attribute. I imagine license, author and prev/next might also be useful for documentation purposes.

[buhtz]

FYI:

• There is an mailman3 (using readme_renderer) issue about it https://gitlab.com/mailman/postorius/-/issues/615
• There is also a hn3 is (by me) asking for an in build solution. feature: clean(..., link_rel=...) adding values instead of overwriting them messense/nh3#71
• Without a hn3 solution I think about a regex based solution/workaround for readme_renderer. But I will come up with details later to discuss them.