[web] alpha layers with DHTML ?

Mozilla has its own CSS style, -moz-opacity and IE has filter:alpha(). There are also some tricks you can do with background images to simulate transparency.

Here's a link that might be helpful
And another
And one more

In fact, searching for "CSS Transparency" will pick up some good links

Yeah, you can do some cool stuff with it. Quite a while back (when I remembered how to do all of that stuff) I wrote a little script to create a pop-up floating menu that approximately looked like a window and could be dragged around, etc...

When you closed it, it smoothly faded out of existence (using timers to measure the delta). It was pretty cool.

Back then it only worked on Explorer, but according to the previous commenter it works in mozilla now. You'll have to determine the browser and generate the page accordingly.