Rounded corners in CSS

Ahh, yes, the elusive rounded corners in CSS design. A number of people have come up with all sorts of hacks and solutions to rounding corners.

Presented is the 'sliding doors' method, where you segment the window into various divs and put an image in each for a more fluid layout.

However, I feel like ultimately, rendering this is the job of a CSS property and the browser. Isn't there a clipping property in CSS?
div#contents {
clip: rect(10px, 5px, 10px, 5px)
}

Now, I only know of shape being 'rect' in CSS2. Why can't there be other shapes, like, 'round_rect'?

Briefly looking through CSS3 spects on W3C, I didn't see anything at all. Does it make sense or no to implement this in the browser, instead of having CSS designers hack up designs?