Yahoo!'s developer CSS libraries are setup specifically with developing with standards in mind. You can be sure if they're using that specific technique there's a damn good reason for it (consider how much traffic the Yahoo network of sites receive compared to your own traffic) - saying you've lost faith in them because a few lines of CSS don't validate is laughable.
Remember they're trying to offer a one-file solution for people to use. They don't have the luxury of adding their non-validating or browser-specific code into seperate files, to be included via CC's.