HTML5 could have taken the extra step of defining a canonical XML or XML-compatible DOM encoding, or at least encouraged it. But the anti-XML sentiment was explicit and spiteful. Still, popular XSLT processors are built to consume and even produce HTML5, I'm just not sure how thorough those capabilities are or whether they're based on any formal or informal standards. (Maybe the W3C has/had something like that, as part of it's attempts to rollup HTML5 snapshots into versioned standards?)

I lost any enthusiasm for XML after my experience with WDDX and XML-RPC, but as a foundation for HTML it was the best thing going. And maybe still is, given the absence of cross-toolchain contenders? Ditto for XSLT, at least if disregarding the choice to make the syntax homoiconic. In fact I circled back to XSLT long after it was clear it was a dead end. A good tool is a good tool, and the fact it remains so despite the wanton neglect only proves it.