> good and working parts are removed
The browser vendors are arguing XSLT is neither good - it's adoption has always been lacking because of complexity and has now become a niche technology because better alternatives exist - nor working, see the mentioned security and maintenance issues. I think they have a good point there.
Well, one can argue that Metafont fonts are "niche". No font library supports them. But from what I know they could easily be technically superior to, say, Type 1 fonts. Of course a technology will be niche if it is treated as a poor relative. XSLT could be an alternative to CSS, for example. It is definitely more powerful than CSS, because it actually transforms the document, not just alters the appearance and sprinkles some automatic content here and there. And is actually used this way in XSL-FO, which, I think, powers a substantial share of technical publishing.
XSLT as an alternative to CSS sounds like a nightmare, though to be fair, I felt the same way about XSLT as an alternative to DSSSL, which to me felt more like a satirical response to the "XML everywhere" zeitgeist in the spirit of INTERCAL and eating babies than a serious design proposal.