Pretty much any open-source language is available on all platforms. Besides, you still need to setup an editor, so it's not like you can skip the whole "install software"-step.
> can be deployed to others easily
In the beginning this doesn't actually matter too much. Beginners are usually aware that they're not creating something that all their friends are going to use, and it's often better to show it to them face-to-face so you can explain things. Sharing code with other students on the other hand is very useful, but JavaScript isn't any different here ("look at my .html" vs "look at my .py").
> a reasonably powerful API (but not powerful enough to shoot your foot)
JavaScript has few built-in libraries, and the DOM API is very HTML-specific and a horrible mess. I don't quite see how these APIs make the students become better programmers; just understand the quirks of HTML more detailed.