Apparently checks CSS and fills up the JS console with errors for CSS that it doesn't like.
Apparently there is no way to disable or filter this whining and it warns on styles that are specific to other browsers like IE. Yes, they are not standard styles, but they are useful for backward compatibility and bitching about them doesn't endear this release to me at all.
Boneheaded thing to do, that. Nice feature - should ship disabled for the average browsing public.