Sometimes you like to think that you pretty much know everything there is to know about URIs and web browsers. I learned something today that I had absolutely no idea about before. The magic data URL protocol.
It’s defined in RFC2397 as a way to include immediate data in a URL.
For example the following defines an inline GIF image in a base64 encoding:

<IMG SRC="data:image/gif;base64,
yAhJZ1/0TxR9dxjeNnkBOFrkvJphu9wAAAAASUVORK5CYII=" ALT="Atlassian Dude">

Which, if your browser supports it, will render thusly:

Atlassian Dude

I only discovered this because Yahoo’s excellent YSlow extension for Firebug suggested replacing small, externally referenced images with data protocol representations.
Pretty neat.

Data URLs