Current Firefox nightlies have support for the bleeding-edge CSS transitions specification. I’ve spent the last few days playing with this feature, and have written some examples as well as reference and how-to documentation for using them.
CSS transitions make it easy to smoothly animate changes to CSS styles, instead of changes taking effect instantly. With a number of ways to control and customize the transition effect, and support for everything from font size and style to colors and even position, you can create some impressive effects with very little work. I suggest taking a look at the demos I put together to get a good idea what you can do.
Obviously since CSS transitions are still in the Working Draft stage, it’s entirely possible the syntax could change, but this is a great way to easily add a little pizazz to your web content.