Hi!
When can I use... seems to think that IE9 supports border-radius: http://caniuse.com/#search=border-radius
At the risk of insulting your CSS experience, you are using border-radius and not the -webkit or -moz versions?
There's a javascript library that might help you out: Modernizer. It's can provide js plugins for functionality that isn't available for a particular version of browser.
HTH
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mark sta ana / @booyaa