I have not used HTML5 since 2009 SP1, but in 2011 SP1 HR1 it worked great for us. I donβt know why this difference would be there. In some cases, the 2011 user interface is, of course, completely different, but I think that the preview was always inside the iframe, which should work fine. If you plan to use 2011, you can also use Chrome and Firefox, so I can certainly recommend the update.
There is one thing we struggled with HTML5 on IE, which is compatibility mode. I never touch the button, but since next to the update button, I noticed that many ppl accidentally turned it on. After that, your browser will return to IE7 mode, which does not work so well for our HTML5 design, even with modernizr. Maybe this is not so much 2009 as your problem, but the compatibility mode that is turned on?
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