In IE9, which was just released, you need to provide the meta tag <meta http-equiv="X-UA-Compatible" content="IE=9" /> to allow the css border-radius to render.
Why?
This link: http://blogs.msdn.com/b/ie/archive/2010/06/16/ie-s-compatibility-features-for-site-developers.aspx indicates that the meta tag is not the only way to enable border-radius. Instead, you can change your doctype.
Read the whole article to determine whether you want to use the doctype or the meta tag, though.
来源：https://stackoverflow.com/questions/5317464/why-does-ie9-require-a-meta-tag-to-render-rounded-corners