Border Radius: Cross Browser Syntax including Mobile with comments explaining CSS support.

index.html

<div id="example">
    Hello!
</div>​

style.css

#example {
    margin: 50px auto; width: 100px; background: #f06; text-align: center; color: #fff;
    
    /* ** START BORDER-RADIUS SYNTAX ** */
    -webkit-border-radius: 20px; /* Saf3-4, iOS 1-3.2, Android 1-1.6 */
    -moz-border-radius: 20px; /* FF1-3.6 */
    border-radius:20px; /* Opera 10.5+, IE9+, Saf5+, Chrome, FF4+, iOS 4+, Android 2.1+ */
    
    /* Stops bg at beginning of border */
    -webkit-background-clip: padding-box; -moz-background-clip: padding; background-clip: padding-box;
}