No doubt! Internet Explorer 9 RC has proved “Why is Internet Explorer the most popular web browser?” because its impact has effected everyone. As Microsoft reported IE9 Beta was downloaded 23Million times since its public release and IE9 RC momentum is still continuing with more than 2Million downloads in first week.
And to keep the same pace to developers, Microsoft has added Internet Explorer 9 Compatibility Cookbook to MSDN Library which is intended for anyone who develops or maintains Internet Explorer applications.
The Internet Explorer 9 Compatibility Cookbook is designed to help you understand changes in Windows Internet Explorer 9 that might impact applications that you developed for earlier versions of Windows Internet Explorer. Many changes help Internet Explorer comply with broader industry standards, and other changes improve performance and reliability.
It also includes information about changes to features, identifies features that are deprecated or removed, and describes general tools and guidance. New topics will be added to this section as features are modified and as you identify areas where you need more information.