www.php.net contains some html-errors. see:
http://validator.w3.org/check?uri=http://www.php.net/
We are continually working on making the home page and the inner pages XHTML 1.0 Transitional compliant. Currently the HTML 4.01 DTD is referenced, but we are using XHTML 1.0 markup in the <head> of pages, and it is impossible to validate as HTML 4.01 this way. If you set the encoding on the validator page to XHTML you will see that we have very few errors left there. Setting the DTD to that on php.net will have a significant impact though, as it affects how the CSS is parsed, how styles are inherited, or what is the modell of the HTML tags in JS, so it needs more test to do this step.
Goba