vim also has completion features documented at http://vimdoc.sourceforge.net/htmldoc/insert.html#ft-css-omni
XHTML5 or polyglot documents
Emacs ships with nxml-mode, a major mode for editing XML which does on-the-fly validation with RELAX NG schema. You can plug fantasai and hsivonen's schema into nxml-mode. The necessary shim code is included in the hober/html5-el project on github.