This is not possible "currently", but there's no technical reason why a browser couldn't provide a proprietary API for this, using non standard html/css/js.
However, that's what it would take, a browser to actually implement this functionality and then expose it as an API, and even then it would be browser specific.
UPDATE (as some people have perhaps misunderstood my answer???):
I'm giving technical context to the question. Of course noone's ever going to implement this, but I'm saying it's technically possible.
Also, doing this would not break the sandbox model. The browser itself (forget an API for a second) could implement transparency any way it wanted. Once it that it could hook it up to it's Javascript engine, and create a stupid call: Chrome.Element("").WeirdTransparency()
UPDATE to Questioner's Update:
to your point:
The reason why would I like it is that
I want to build a cross-platform
(Linux, Windows, Mac), zero-install,
fancy-looking rich client application
(not meant to be served as an Internet
website)
AIR kinda covers 90% or your requirements. It still needs a small install, but apart from that, you're running...
