cross-fade is a CSS function from the CSS Images Module Level 4 spec. Per a suggestion from dholbert, I'm making this bug to track its overall implementation. There are essentially three parts to this:

1. Parsing for cross-fade.
2. Allowing multiple image requests to be associated with one image in Gecko (think cross-fade(url(example.com/fish.jpeg), url(example.com/bicycle.jpeg))).
3. Sizing and graphics for cross-fade.

Parsing is complete and currently merged into central as part of bug 546052, associating multiple image requests is being tracked by bug 1653703, and I'll make a new bug for the graphics part.

Resolving this bug will be a matter of completing those tasks and then enabling the layout.css.cross-fade.enabled preference by default.

Graphics and sizing being tracked by bug 1657517.