At the moment, the only fully supported language, and the de-facto standard for DOM tree manipulation in the browser is JavaScript. It looks like it has deep design issues t
Internet Explorer supports pluggable scripting languages, although the only one reliably included with IE besides JScript is VBScript.
As far as I have seen, there seems to be a general sort of bias toward dynamic languages in the browser, and JavaScript seems to fill this need adequately enough that network effects make any other language a non-starter. The language is actually quite powerful, though its implementation in browsers leaves much to be desired.