Sorry for some reason I can't reply to the deeper posts. Depth limit I guess. You asked which editors could support syntax highlighting once the tag and the content that needs to be syntax highlighted are split apart. Currently there are none, though I am writing one that will be able to.

Python should be able to detect the magic syntactic pattern just the same way JS does and use that for syntax higlighting. In JS the magic syntactic pattern for triggering HTML syntax highlighting is:

  html`<doc/>`

In Python the magic syntactic pattern would be:

  html(t"<doc />")

My point in showing that JS counterexample was to demonstrate that the real reason people don't do that kind of thing isn't that they can't, it's that they like having syntax highlighting. That means the approach should work just as well in Python. This is the case even though the heuristic involved is very weak. Changing the name of the identifier or breaking into multiple expressions would be enough to break the heuristic in either language, which is why I think it's a really weak heuristic and dangerous pitfall for developers who might mistake the coloring for information about how the runtime sees that data (which is normally what syntax highlighting is for)