It’s actually worse than XSLT 1.0 due to inconsistencies and incompletenesses. For example, Firefox doesn’t respect <xsl:output method="html">, but uses an XML parser on the transformed result regardless; and doesn’t support disable-output-escaping. I wanted these for my Atom stylesheet (for <content type="html"> and the likes; instead I had to emit serialised HTML and decode it in JavaScript, though with difficulty I could have done feature-detection to skip that step if disable-output-escaping worked).

Even perfunctory probing shows fairly serious problems in Firefox (where Chromium is consistently much better, in this specific area). I could file quite a few bugs in short order (e.g. these mentioned, bad document.contentType values, <template> not working properly), but I don’t think there’s any interest in fixing things.

(I wrote this comment as much for my own future reference as anything else. XML/HTML polyglot stuff makes things decidedly messy at times.)