This project provides the ability to use nxml-mode to edit XHTML5 documents with validation. First, you'll need to fetch the RELAX NG schema from the WHAT Task Force subversion repository; a simple make relaxng in this directory should do the trick.

Next, you'll need to add this directory to your load-path. Let's assume you've placed it in ~/code/html5-el/.

(add-to-list 'load-path "~/code/html5-el/")

Finally, wire this stuff into nxml-mode like so, altering the path to schemas.xml appropriately.

(eval-after-load "rng-loc"
  '(add-to-list 'rng-schema-locating-files "~/code/html5-el/schemas.xml"))

(require 'whattf-dt)