Do you foresee cases for when html-ts-mode would be preferred by the
user instead of this advanced mhtml-ts-mode?
For everyday use mhtml-ts-mode is better, just like mhtml-mode (which has been
the default for html editing for a while now).
Or maybe the former is
better in its current shape when used by e.g. php-ts-mode?
Yes, personally I think that major modes that handle (for tree-sitters) only
one language are easier to put together at the moment. It's Lego vs.
Playmobil.
We are in an experimental phase, like all other editors.
See https://github.com/helix-editor/helix/pull/1170#issuecomment-997294090
In some ways, by having a different approach from other editors, we have a
greater degree of flexibility IMHO.
In other words, I'm wondering why not update the existing mode with
sub-parsers rather than add a new one. html-mode had such a reason -
it's quite old, and has been used in various placed the way it is now
(including multi-mode packages). But ts modes don't work too well with
multi-mode packages, not currently anyway.
It's something I've thought about but haven't tried yet.
One of the themes of the email thread (on emacs-devel) was to have a simple
multi language major mode that was also a sort of “user's guide.”