Why is it that so many CMSs make it harder than it should be to use the markup you want to? And why do CMSs still, in the year 2010, output cluttered/inaccessible/outdated HTML that can’t be easily overridden? There may be exceptions of course – I haven’t used all CMSs that exist. There are hundreds of them, so I doubt anyone has.
A CMS should never, ever output HTML that is beyond the control of the developer.
The robustness of a CMS is bottlenecked by the ability and clout of its developers.