I generally get rounded corners just with css, if browser does not support they see the content with flat corners. If rounded corners are not so critical for your site you can use these lines below.

If you want to use all corners with same radius this is the easy way:

.my_rounded_corners {
  border: 1px solid #ccc;
  -moz-border-radius: 5px;
  -webkit-border-radius: 5px;
  -khtml-border-radius: 5px;
  border-radius: 5px;
}

but if you want to control every corner this is good:

.my_rounded_corners {
  border: 1px solid #ccc;

  -moz-border-radius-topleft: 10px;
  -khtml-border-top-left-radius: 10px;
  -webkit-border-top-left-radius: 10px;
  border-top-left-radius: 10px;

  -moz-border-radius-topright: 0px;
  -khtml-border-top-right-radius: 0px;
  -webkit-border-top-right-radius: 0px;
  border-top-right-radius: 0px;

  -moz-border-radius-bottomleft: 4px;
  -khtml-border-bottom-left-radius: 4px;
  -webkit-border-bottom-left-radius: 4px;
  border-bottom-left-radius: 4px;

  -moz-border-radius-bottomright: 10px;
  -khtml-border-bottom-right-radius: 10px;
  -webkit-border-bottom-right-radius: 10px;
  border-bottom-right-radius: 10px;
}

As you see in each set you have browser specific styles and on the fourth rows we declare in standard way by this we assume if in future the others (hopefully IE too) decide to implement the feature to have our style be ready for them too.

This works beautifully on Firefox, Safari, Camino, Chrome. For other browsers there are tons of other tweaks you can find by googleing.