Here is W3C validation service: http://validator.w3.org/
Enter plain vanilla BE 2.5 site: http://dnbegallery.org/preview2
The document located at <http://dnbegallery.org/preview2/> was successfully checked as XHTML 1.0 Transitional. This means that the resource in question identified itself as "XHTML 1.0 Transitional" and
that we successfully performed a formal validation of it. The parser implementations we used for this check are based on
OpenSP (SGML/XML) and
There is not much to fix, if you not getting validated it is probably because theme or post content has invalid HTML. Could also be something in blogroll or injected by extension.