Right now, the CSS property parsing code mostly:

 - Silently ignorest hings it doesn't understand
 - Warns when it hits something that's not supported

(Though there are some other warnings.)

This is going to be hard on someone doing CSS work - we probably should consistently warn any time we know we're not doing what the stylesheet author intended.

If we allow theme development by writing stylesheets, it's probably nice to collect the errors some other place than the stderr of the gnome-shell process. 
(Same thing applies to extension/plugin development by writing JS). But just warning to stderr is probably a good start.