I think this places too much of a limit on the designers. I like xhtml, but somtimes I used HTML 4. Well formed mark up is great, but I might want to use IEs -1 value for tabindex in forms. That won't validate under any HTML 4.01 versions.

Your designers will love templates. They can play with new designs without bothering you guys while still using curerent data. And your code looks much cleaner when using templates than when using embedded markup.