User blog comment:DaNASCAT/Technical Update: March 20, 2012/@comment-1513877-20120324033637/@comment-2200849-20120326132529

Gradients are part of the CSS Image Values and Replaced Content Module level 3, which is a W3C Working Draft as of January 12, 2012. Since the fate of the document is unclear, developers provide support for the contents with vendor–specific prefixes, which will disappear if the draft is deleted or approved. The same happened to CSS Backgrounds and Borders Module Level 3; after it became clear that the  property will be included in the final variant, Mozilla ceased support for. And you kill a kitten if you call CSS3 proprietary.