[mgr86]

I'd argue XHTML did take off and was very widely adopted for the first 5-10 years of the century.

[pests]

From a recent HN post, ePub's are technically XHTML by spec.

[dragonwriter]

EPUB 3.x spec uses “XHTML” to refer to the XML serialization of HTML in the current WHATWG HTML living standard, not the separate historical standard from HTML known as XHTML.

The obsolete EPUB 2.x (and earlier, I believe) specs actually used XHTML (XHTML 1.1 in EPUB 2.x), though.

[mgr86]

Yes, that is an important point. I guess I was focused mainly on the browser. Which is not the typical way to consume epubs. But you are right that ePubs are essentially packaged XHTML.