FWIW, the KompoZer codebase seems to have been ported to Mozilla's gecko browser and so lives on in some form and says it supports xhtml5.
https://github.com/therealglazou/bluegriffon
It is supposed to run on all platforms but I do not see any release binaries.
Postscript: wow their epub editor features require a 195 euro license fee!
Ouch!