As Mambo 4.6 beta is announced and running on
http://www.mamboserver.com/ I decided to give it a try and run the w3c validator on that site. I've heard that the xmambo developers (who created an xhtml compilant version of mambo) have joined the mambo core team, so I expected the new version to validate.
So I was quite disappointed that Mambo 4.6 beta is not one dot better than mambo 4.5 concerning xhtml compatibility.
The recently announced typo3 3.60 produces valid xhtml now, but it is harder to work oneself into it.
The version 5 release is still far away, so the only short term solution to create validating code is to use xmambo, but which may be incompatible with existing modules or components...