Very fortunately, albeit lately, rounded corners attributes have been added to CSS. CSS3 contains the attribute border-radius (e.g. border-radius: 15px;) which makes doing rounded corners on divs a breeze.
Unfortunately, this is not backwards compatible with many browsers created before this addition, including all Internet Explorers before IE 9.
To put rounded corners on an element that works on all browsers, past and present, create a class as below.
You need to download the above .htc file from curve-corner at google code.
You can change the radius above as you wish. There are some obscure situations in IE where above doesn’t work. Ways to handle this are at the link here.