Only use [b]Strict[/b] Doctype headers when producing HTML 4.01 and XHTML 1.0 web pages. If you use [b]Transitional[/b] Doctypes you are incorporating deprecated Markup and your pages may render in Quirks mode thereby inviting problems.
Remember that if you serve XHTML as content (MIME) type [b]text/html[/b] (as 95%+ presently is) you are actually only producing HTML pages – if you wish to produce them as true XHTML (with XML functionality) pages you need to serve them as [b]application/xhtml+xml[/b] – see [url=http://jp29.org/test.php][u]Problems Serving XHTML 1.0 Web Pages[/u][/url]