The W3C CSS Working group has posted a working draft of CSS Image Values Module Level 3. "In CSS Levels 1 and 2, image values, such as those used in the ‘background-image’ property, could only be given by a single URI value. This module introduces additional notations that allow a 2D image to be given as a list of URIs denoting fallbacks, as a slice of a larger image (sprite), and as a gradient."