Back in October I opened up a MS Connect feedback item about a validation error I was receiving with a XHTML Strict site that used a ScriptManager with history support. The site would fail W3C validation because the ScriptManager emits an IFRAME (when history is enabled) , and an IFRAME is not part of the XHTML Strict spec. I just thought I’d provide the official word from Microsoft on this issue: XHTML Strict is not supported by ASP.NET!
XHTML strict is not supported by ASP.NET. Only transitional doctypes are supported, which enables iframes. Furthermore, the history feature can't be made compatible with IE 6 and 7 without using an iframe.
Posted by Microsoft on 11/12/2008 at 12:34 PM