XML is a failure compared to the totally unjustified hype about how it would replace all other data formats and lead us into a new computing nirvana. Compared to the goals of its creators, XML is quite successful.

He's talking about XML on the web, not XML in general. XML is not a failure in general (although it should be...).

XML is incredibly powerful, somewhat complex, and therefore misunderstood.

It needs an "Modern XML" like the "Modern Perl" that's going around - a take from someone who has used it to solve problems and goes over the bright points and black marks.

For example, to the neophyte, do you pick DTD, XML Schema, RelaxNG, or Schematron to solve data validation task X? How about task Y?

Too many people tried to use XML as a CSV or other structured data replacement, where JSON is usually superior. I doubt that anyone thinks that JSON is a HTML replacement, but XML can be.

It comes down to different tool, different job, and using the wrong tools can leave a sour taste in one's mouth about a technology - how many people here have similar feelings about Java or Windows?

I don't think it's fair to call XML a failure any more than you can call Perl a failure. XML does the job, and it's still used in a lot of places. It's just that it's being supplanted by a newer technology that is superior for the majority of use-cases.

XML is being abandoned as the medium for a variety of APIs at the moment (Google and Facebook especially). It looks like failure when my XSLT code deprecated before I can even release it.

umm do you mean "superior" in regards to capabilities or regards to simplicity. You could just as well argue that JSON replacing XML is akin to XML replacing SGML.