We have <!doctype html> but why not add other doc types as time goes on? Instead of shoehorning features you could have a <!doctype new-html> which would opt a web page into the ability to use newer features like newer elements or cleaner APIs
There is also the “use strict” directive in JS (which any script tag with type=“module” runs in now, slowly eliminating this need) that would opt your script into a stricter form of JS. You could even do it on a per function basis. We could have something like that for other features or stricter qualifiers etc.
We have done this before successfully and broadly, I think it’s time we revisit the usefulness of doing this again