One of the best sites i came across was http://www.quirksmode.org/
http://www.quirksmode.org/compatibility.html
Compares major browsers’ CSS and JavaScript capabilities, as well as their adherence to the W3C standards.
Added 18/01
When can I use... http://caniuse.com/
Compatibility tables for support of HTML5, CSS3, SVG and more in desktop and mobile browsers