Detect "costly" CSS

[dontcallmedom]

Some CSS properties can result in worse performances; the checker should draw attention to this when it detects they're in use.

Part of this issue is to collect with of these properties ought to be warned about; off the top of my head:

• animating properties that can't be GPU optimized (see http://www.html5rocks.com/en/tutorials/speed/high-performance-animations/ )
• drop shadows