Please see my comment to the question.
My idea is: take a good rounded corner generator
. You will find a plenty of them: http://bit.ly/POC2pp
I used this one: http://cssround.com/
The aspects of browser compatibility are explained here: http://www.css3.info/preview/rounded-border/
Don't trust it fully, better check up with browser documentation on each well-known browser.
It will give you the cross-browser code (if a generator is good). Now, choose
height exactly twice as radius
. If you want a border, slightly change the size, by the size no bigger then border width. You are done.