Here are two very useful sites :
http://www.caniuse.com/
Like it says, it shows “compatibility tables for features in HTML5, CSS3, SVG and other upcoming web technologies”.

Another one is http://fmbip.com.
It displays a table showing exactly what browser you’re using and what CSS3 & HTML5 capabilities the browser supports.
http://fmbip.com/litmus displays test results showing when you can start to use specific HTML5 & CSS3 functionality.

Very handy resources for developers !