Hier ein Beispiel der W3 Prüfung, es scheitert schon am simpelsten überhaupt und die seite läuft trotzdem mit tausenden klicks pro tag...tja auch so kanns gehen:
Warnings
Warning: No Character Encoding detected! To assure correct validation, processing, and display, it is important that the character encoding is properly labeled. Further explanations.
Below are the results of attempting to parse this document with an SGML parser.
Fatal Error: no document type declaration; will parse without validation
I could not parse this document, because it uses a public identifier that is not in my catalog.
You should make the first line of your HTML document a DOCTYPE declaration, for example, for a typical HTML 4.01 document:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<HTML>
<HEAD>
<TITLE>Title</TITLE>
</HEAD>
<BODY>
<-- ... body of document ... -->
</BODY>
</HTML>
Sorry, I can't validate this document.