@ifingers: HTML validation in general definitely is a good thing and our themes also are W3C valid as we usually test this ourselves.
But often people take this too serious and then contact theme developers just because something on their site isn’t W3C valid and then get crazy about this and that’s the scenario which isn’t worth it as a few minor W3C notices isn’t an issue, especially as it’s sometimes even necessary to do things that aren’t W3C valid but make sense in other ways.
With regards to Felin’s validation error. It looks like this is due to the validator being incorrect at this point of time. So, probably safe to ignore.
Yes, that’s why we mentioned it. There is no need to worry about things that aren’t to worry about. In general W3C notices like this are related to plugins, 3rd party scripts or in this case WordPress itself and there is a reason why it’s coded like that. ??