Web assembly could allow dynamically-typed Python to become a first-class citizen, if someone were inclined to write a Python interpreter stack that compiled to LLVM (which might already exist? https://github.com/dropbox/pyston)

The purpose of WebAssembly, pNaCl, and its ilk, is to get out from under the unfortunate accident that the "assembly language" target for browsers---i.e. the symbol set the browser can directly interpret and translate into machine operations---is JavaScript, which is not a language designed with memory or runtime performance deeply considered (or type safety, for that matter). While there are projects that somewhat ameliorate this issue (TypeScript, for example, tries to add type safety to the language), writing a something-to-JavaScript compiler is a much taller order than writing a something-to-LLVM compiler, which can then be used by an LLVM-to-WebAssembly compiler---letting you ultimately write your web site in any language you are fluent in.