box-shadow is part of CSS 3. It has been supported in Safari since version 3.1, and is supported in Firefox 3.5, Chrome and Opera 10.5. Safari, Chrome and Firefox require the browser prefixes -webkit- and -moz-, but Opera does not.
CSS Shadows take three length values, and a color. The length values are a horizontal offset, a vertical offset and a blur. offsets may be negative or positive