What's with the amount of incredibly confident misinformation here? If you don't KNOW, don't comment. JPEG absolutely supports transparency - transparency is a component/channel, and JPEG can support thousands of channels. JPEG is color-blind, it doesn't even know about red, green, and blue channels, or YCbCr channels, without a JFIF or SPIFF wrapper. Just as JPEG can support 12-bit channels, not just 8-bit - doesn't mean a browser knows what to do with the data. The issue is whether a given browser can decode and properly interpret a channel as meaning 'transparency' during its rendering.
