I just noticed this earlier today.. When I run the default theme index.css through the validator it has three errors.
I have a picture attached below to show it. Now here is my question, why is this? I know in order to submit a theme and get it accepted you have to have valid html? Which I think is a good idea. However why is this not true for CSS and why does the default theme have invalid CSS. Making the in question code valid is not hard at all.