Actually, WordPerfect has supported XML for years (Score 2, Interesting)

...Of course, not very well--but it's pretty easy to compile, say, the Docbook 4.1 DTD in Wordperfect and edit moderately complicated documents. Or import... The limitations are that it uses its own formating system, rather than XSLT; and it uses DTDs instead of schemas, because the technology derives from SGML (which wordperfect also supports). Arguably, WordPerfect has better support than any of the alternatives within the word processing space (i.e. discounting pure editors such as EMACS).