But the fundamental problem here is the same: no matter what new things are added to the spec, the best you can hope for in browsers is XSLT 1.0, even though we've had XSLT 3.0 for 8 years now.

KDL is a very interesting attempt, but my impression is that people are already trying to shove way too much unnecessary complexity into it.

IMO, the KDL's document transformation is not a really good example of a better XSLT, tough. I mean, it's better, but it probably can still be improved a lot.

That's YAML, and it is arguibly worse. Here's a sample YAML 1.2 document straight from their spec:

    %TAG !e! tag:example.com,2000:app/
    ---
    - !local foo
    - !!str bar
    - !e!tag%21 baz

--

Some notes:

- SemWeb also has JSON-LD serialization. It's a good compromise that fits modern tooling nicely.

- XML is still a damn good compromise between human readable and machine readable. Not perfect, but what is perfect anyway?

- HTML5 is now more complex than XHTML ever was (all sorts of historical caveats in this claim, I know, don't worry).

- Markup beauty is relative, we should accept that.

Trying to close the gap often ends up creating more complexity than intended, or maybe even more than XML in some hands.

It definitely would be an interesting piece.