Except for creating an HTML-file and understanding how all that works.
> and the OS becomes (to a degree) irrelevant.
This is pretty much true for any language as long as you don't use platform-specific features (fork).
> Further, javascript is an easy language to prototype in, and, at the basic level, easy enough to teach and write.
"Easy language to prototype in"? Both Python and Ruby are equally "easy", but comes with even more batteries included. They also have a consistent object model, as opposed to JavaScript's prototype-inheritance-is-cool-but-let's-mix-in-constructors-from-classical-OOP. There's pretty much nothing like JavaScript's prototypes in any other languages (Self/Io works quite differently), and I don't really see any gain in teaching that to students early on.