CSS
-webkit-border-radius
-moz-border-radius

It's a tough decision to start dramatically forking the experience between IE and “the rest of the world.” But, before you get overly excited, note the border-radius has it's gotchas!

1. FF 3.0 doesn't support individual corners Corrected - thanks Brainlag

   Yep, I'd really recommend squaring the diminishing world of FF3.0 as well. By all means, ROUND EM IN FF3.0! This can be done by targeting FF3.0 and above with @media queries. This will require a constraint since @media partially worked in FF3.0. So, try @media (min-width: 1px;) { } corrected: @media screen { #myDiv { -moz-border-radius: 5px; } }

2. Background images spill out of rounded boxes in FF3.0

   I believe FF3.5 properly masks background images finally

3. Rounded corners in FF2.0 aren't anti-aliased

   This is probably not much of a problem given FF2.0's 3% market share.

These quirks are no cause for concern. In fact, I think you should leave FF3.0 and pre Safari 4.0 out of the rounding world by the fact that older browser FF and Safari users disappear faster than “a truckload of dead rats in a tampon factory!” - gratuitous, meaningless quote from Top Secret.

What do you guys think? Any major CSS Rounded Corner converts?