[CSS21] HTML examples should use HTML 4.01 DTD

Most HTML examles in the WD use HTML 4.0 doctype, some less (2 I think)
use HTML 4.01.
To avoid spreading bad practise to the web, only HTML 4.01 should be used.
HTML 4.0 DTD is not recommend to use:
http://www.w3.org/QA/2002/04/valid-dtd-list.html