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My company is building an App Engine application. The app encounters a bug (possibly due to an issue with App Engine itself, as per our research) on IE9, but it cannot be reliably reproduced and is experienced by a small percentage of users.

The workaround is to force IE9 to use IE8 mode. As a lazy front end developer (who doesn't like CSS hacks, shims and polyfills) I think it's OK to at least try going back to IE9 mode and see what happens, while we're still in private beta. The senior engineer (being more pragmatic) would rather that we continue forcing IE9 users to use the older IE8 mode. Who is right?

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For those of us who don't know, are you doing something in your app that enforces IE8 mode, or are you instructing your users to manually set IE8 mode? – Jeremy Heiler Jun 1 '12 at 14:57
Using a meta tag to instruct IE9 to render as IE8 – Junior Dev Jun 2 '12 at 0:52
What is the specific issue you're facing? IE8 was less standards compliant than IE9. Is your markup valid if you run it through the validator? -> – Ryan Hayes May 10 '13 at 19:20

If your code is standards-compliant, but IE9 breaks unless put in IE8 mode, I'd say keep it as it is. Just make sure you're not breaking anything for other browsers for the sake of salvaging IE. You may get into trouble when you try to use standard functionality that isn't available on IE until version 9 though; if that happens, you should re-assess and decide what is more important - the bug not appearing, or full functionality when it doesn't appear.

If you can pinpoint the problem to a documented bug in IE, you may want to implement a workaround (again, one that doesn't affect other browsers) or just switch back to IE9 mode once the bug is fixed (plus a bit of a grace period, because not everyone updates their systems on a daily basis).

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