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I have successfully tested the functionality of the AccessKey of RadButton under IE8. I have also attached a short video and the test page from VS in the archive - The AccessKey attribute is applied to the rendered RadButton as well as to the Input Button and it is working properly.
You can make a test with my sample and if the problems persists then this behaviour might be related to the browser. Therefore you can try to reinstall it, clear its cookies, or disable your plugins which could affect the proper functionality of the AccessKey.
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