Here's a small example on how to pass W3C validation while enjoying Dojo's de facto standard of mixing-in new attributes. This approach leverages XHTML1.1 Modularization http://www.w3schools.com/xHTML/xhtml_modules.asp.

W3C Validating XHTML: http://yadtd.info/dojo-release-1.1.1/dijit/demos/formw3c.html
Original HTML: http://archive.dojotoolkit.org/dojo-2008-07-01/dojotoolkit/dijit/demos/f...

Differences (HTML formatted): http://yadtd.info/dojo-release-1.1.1/dijit/demos/formw3c.diff.html
Differences (diff -u): http://yadtd.info/dojo-release-1.1.1/dijit/demos/formw3c.diff.txt