[dekhn]

At the time HTML was converted from SGML to XML: https://en.wikipedia.org/wiki/XHTML so if you authored XHTML, you could XSLT it. There is also XHTML5, an XML serialization of HTML5. I imagine in the real world there is a great deal of web that is HTML, accepted by browser, but not XML.

[kbolino]

As far as I know, HTML5 has diverged from its origins enough that it's neither SGML nor XML. However, given the existence of XHTML5, it might be possible to parse an HTML5 DOM and re-serialize it as XHTML5, and thus it might be possible to take parseable HTML as input to XSLT, albeit with some indirection.