you should then try http://liveweave.com. they have code-hinting for HTML5, CSS3, JS, jQuery...really awesome stuff. I recently found it, and loving it so far.

http://liveweave.com is by far the best HTML/CSS/JS playground out there. you guys should totally try it out.

I can name three problems with it straight-away: 1) HTML, 2) CSS, 3) JS.

The original post is about a decent language, not the ubiquitous ones.

Honest question: Why?

I think I can answer that. IMHO Liveweave has something most of the other JS "playgrounds" do not have. It has context-sensitive code-hinting for core JavaScript and jQuery...also for CSS/HTML/HTML5/CSS3. Personally, I think this makes a huge difference, at least for me and may be some others.