Use http://html5please.com to see what's "dangerous" and don't even think about coding CSS by hand (ie use Stylus, SASS, or Less to serve all vendor-specific flags).
Don't use vendor prefixes.
We can also demand vendor free CSS from our contractors and developers. I've had to ask several contractors, usually younger developers, to please not use prefixes. They comply, but clearly think I'm too old school because CSS 3 is what all the cool kids are doing.
To be clear... are you saying you ask contractors to not use features like CSS rounded corners, gradients, and box shadows?