css2 border-radius：用图片
css3 border-radius: firefox3.5+ safari3.0+ chrome 4.0+ opera 10.5+ ie 9+
w3c: border-radius: <length>{1}：4角相等， <length>{2}交叉相等， <length>{3}右上左下相等 , <length>{1,4} / <length> {1,4} 前面水平半径，后面垂直半径
border-top-left-radius/border-top-right-radius/border-bottom-left-radius/border-bottom-right-radius: <length> <length>
firefox4 safari5 chrome 10 opera 10.5 ie9
-moz-border-radius-topleft: <length> <length> //左上角
-moz-border-radius-topright: <length> <length> //右上角
-moz-border-radius-bottomright: <length> <length> //右下角
-moz-border-radius-bottomleft: <length> <length> //左下角
-webkit-border-top-left-radius: <length> <length> //左上角
-webkit-border-top-right-radius: <length> <length> //右上角
-webkit-border-bottom-right-radius: <length> <length> //右下角
-webkit-border-bottom-left-radius: <length> <length> // 左下角
border-top-left-radius: <length> <length> //左上角
border-top-right-radius: <length> <length> //右上角
border-bottom-right-radius: <length> <length> //右下角
border-bottom-left-radius: <length> <length> //左下角