How come the default theme Twenty Eleven fails validation test, both for HTML5 and CSS3? I checked: http://twentyelevendemo.wordpress.com/
Should I be bothered that the validation fails?
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Validation is a tricky topic. It is good to have, but it is also theoretical thing in practical world.
It explicitly says that HTML5 checking is not considered reliable and errors are mostly attribute related (which is verrry unlikely to be an issues).
CSS errors seem to be related to multiple browser-specific feature. Annoying? Yes. Possible to get by without? Will need to sacrifice some eye-candy.
My opinion is that unless you are dead set to only using perfectly validating themes on principle, then Twenty Eleven is not for you or you will need to create child theme and clean it up.
If you are concerned that Twenty Eleven just might have horrible markup/CSS issues - that is unlikely in theme of such high profile.