This issue is caused by IE 6.0 own built-in bugs.

 

 

If you choose to use an XHTML doctype , please note that the XML declaration makes IE 6 (but not IE 7, IE 8.0) trigger the Quirks mode. All Standard compliant browsers – including IE7 – will read the DocType Declaration and interpret our markup and CSS accordingly. That's the whole idea, as it is only IE6 that is too weak with regards to W3C standards.