What is -webkit or -moz?

Browser makers like Apple, Mozilla, Opera, and Microsoft sometimes add an extension to the beginning of a CSS property to indicate that property only applies to their browser. Developers use this technique when they want to add a CSS property that isn’t part of the CSS standard.

Properties beginning with -moz- are for Firefox, -webkit- is Safari, -ms- is Internet Explorer, and -o- is Opera. Usually, if the W3C CSS Working Group adopts the property and finalizes enough of its details, vendors drop the extension. For example, Firefox now understands the CSS 3 property opacity, but earlier versions of the program used the property -moz-opacity.