[sametmax]

Oh we wish. People tried (brython came close). But any attempt are currently not practical as the runtime is very heavy, not to mention the stdlib.

So we are stuck unless the browser vendors decide to ship a Python VM next to their JS VM. And they won't do that.

[fulafel]

ClojureScript does very well despite the parent Clojure having a big runtime and largish stdlib. Good tree shaking (dead code elimination) helps a lot.

[paulddraper]

Specifically, tree shaking through the venarable (and somewhat confusingly named :) Google Closure Compiler.