I write a lot of javascript that has to run perfectly down to IE7, and this article is a good starting point for anyone seriously focusing on cross-browser compatibility. One thing to know is that it's not always possible to use polyfills -- for instance, there is no good shim for columns in IE < 10. That said, if there is a polyfill available, Modernizr's conditional loading is a great tool to use.
Really appreciate this series. I always find a little something I didn't know or forgot about. Thanks.