"supposed" would be a better fit than "poised".

> much more than "correctedness"

Not really

> Reliable validation, custom DTDs

correctness.

> extensions

Really debatable, xhtml only added "extensions" in the "XML dialects, RDF!!!" sense, in the effective sense HTML is extended all the time.

> better interoperability

At the cost of interop with the real world. And that better interop was restricted to markup (an issue being much better solved through HTML5 as browsers are moving towards spec-compliant HTML5 parsers), it left the "new" interop issues (CSS, JS, DOM, ...) in place.

And XHTML offered this better interop by... mandating source correctness...

> It just wasn't meant for the general web, where you have a massive number of non-technical authors.

Which, interestingly, is also an issue with Python: it's used a lot in scientific fields and as an extension language (though less so than it used to be), which I'd guess would qualify as "non-technical authors" as they're not computer technicians.

You just retold the whole of his Python 3 argument in terms of XHTML. P3 was also poised to offer more than "correctedness", and also "wasn't meant for the general real world where you have a massive number of different systems".

"""I don't think this comparison has any place in this discussion."""

Actually it's the perfect analogy.

XHTML -> add correctness, some new features, idealistic, unsuitable for the real world, didn't catch on.

Python 3 -> add correctness, some new features, idealistic, unsuitable for the real world, didn't catch on.