The shortened doctype looks odd, but is the new official recommendation for HTML 5. Koha has changed to this for the OPAC and (I believe the staff side).
The above is 100% correct, as were the other replies you have received.
Yup, but I tried that (explicitly declaring HTML5)
"<!DOCTYPE html>" explicitly declares an HTML5 document.
of errors (50+ of them yui css)
YUI CSS is not HTML, and thus are completely irrelevant to questions about XHTML vs. HTML5. The W3C's markup validation service will correctly validate HTML5: http://validator.w3.org/ To sum up: The doctype declarations in Koha's OPAC and staff client templates are correct, and are not the cause of your errors. Not all templates are 100% valid, but you should see few errors related to HTML markup. -- Owen -- Web Developer Athens County Public Libraries http://www.myacpl.org