HTML5 text/html syntax
https://vim.svn.sourceforge.net/svnroot/vim/vim7/runtime/autoload/xml/ and :help xml-omni-datafile might help you get started.
Ideally some sort of tool to convert http://syntax.whattf.org/relaxng/ into a xml-omni-datafile is required. Updating https://vim.svn.sourceforge.net/svnroot/vim/vim7/runtime/autoload/xml/html40s.vim looks a bit too painful.
See the HTML5 conformance checking in Vim blog entry for an introduction.
- HTML validation with validator.nu script
- CSS validation is being worked on
Any suggestions? Please get in contact with Kai Hendry
- Vim's makeprg syntax requires some horrible escaping, hence the need for an external script
- Vim's errorformat has issues parsing anything too complex, so best edit the stream for the errorformat (not the other way around)
- Vim's quickfix list (:cope) seems to require a filename (%f) in the error format
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.