Actually if your talking about XHTML compliance, Mambo and its default templates have been XHTML compliant since 4.5.1
However, some free templates are not xhtml compliant and also once you start entering content, most of the wysiwyg editiors have trouble creating xhtml compliant data.
Note, the use of tables for output rendering is not illegal in regards to xhtml compliance.
However, the reason why people are moving away from tables for output rendering is outlined here:
http://forum.mamboserver.com/showthr...ght=css+tables