As Sandeep correctly put it, rounded corners are not supported by all browsers, but you can always design CSS to support as many browsers as possible. You can find nice explanation and the overview on different browser support here:
http://jonraasch.com/blog/css-rounded-corners-in-all-browsers[
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See also:
http://www.css3.info/preview/rounded-border/[
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It is very convenient to use CSS Border Radius Generator:
http://border-radius.com/[
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[EDIT]
By the way, in all designs its would be the best to provide a simplified fallback design; for the cases when a Web browser does not support some feature at all, the view should gracefully degrade, still having a decent readable look.
—SA