Compat Inspector is a JavaScript-based testing tool that analyzes your site while it runs. Compat Inspector reports patterns of interaction which cause issues in standards mode. This allows you to identify problems quickly without memorizing a bunch of documentation or searching through the entirety of your site's code. We developed Compat Inspector during the course of IE9 to speed up the process of recognizing common issues across different sites. Along the way many members of the IE team contributed to the set of test cases that make up the rules in Compat Inspector. Now we're making it all available to you. Check out the Compat Inspector Test Drive to try it out before using it on your own site.
Source : IEBlog
