A More Compatible IE9, Through Your Feedback

April 1st, 2011

Throughout IE9, we have written about how users and developers expect browsers to be compatible with the sites they use every day. This blog post covers a few ways we measure and ensure compatibility with the Web and how your feedback made a difference.

Measuring compatibility with the Web

We measure Web compatibility through many different mechanisms. These include:

  • Customer feedback: Most important, we listen and act on feedback from the community. With 33 million downloads of IE9 Beta and RC, users logged over 23,000 bugs. That’s over six times the amount of feedback we received in previous versions (Users logged more bugs for IE9 in part because we made it easier by opening the Connect program to everyone).
  • Professional testing: We test thousands of high traffic sites on a regular basis. These sites are popular with users and important to daily life around the world.
  • Telemetry: Across hundreds of billions of browsing sessions, we gather telemetry data when users click the Compat View button. We use this data to determine what additional sites to test and add to the Compatibility View (CV) List.

We use all of this data, analysis, and testing to improve IE9’s compatibility with the Web. We also use it to determine what sites are added and removed from the CV List. For example, thanks to your feedback, we removed hundreds of sites that have updated since Beta such as and This helps ensure that all users have the best browsing experience with these sites.

New for the IE9 Compatibility View List

We made three improvements to the IE9 CV List between Beta and the final release:

  1. The IE9 CV List includes the software fallback list for GPU/drivers, which you can find at the end of the CV List xml file. This means we can update the CV List to provide users with a better browsing experience within days of discovering GPUs or drivers that are incompatible with IE9.
  2. The IE9 CV List includes subdomain support. This enables IE to run a specific subdomain in a compatible document mode, while keeping the rest of the domain running in the latest Standards mode.
  3. F12 Developer Tools tells site developers when sites are on the IE9 CV List, their local CV List, or a Group Policy-configured CV List, as shown in the screen shot below.

Ensure the best experience for your site

In closing, please continue to verify your site works well with IE9. Start by sending IE9 the same standards-based markup and script you send other browsers, and use feature detection, not browser detection. Check to make sure your site is not on the CV List. If it is, that means customers have reported incompatibilities with your site and IE9. To be removed from the CV List, test your site with the CV List off (Press “F12” to bring up F12 Developer Tools and select “IE9” from the Browser Mode menu), and update your site to be compatible with IE9. When you have verified that your site works in IE9 Browser mode, email with the information at the bottom of this Web page and ask for your site to be removed from the IE9 CV List.

—Marc Silbey, Program Manager, Internet Explorer