Michael Key, author of the Saxon xslt processor, being inspired by the GWT ideas, has decided to compile Saxon HE into javascript. See Compiling Saxon using GWT.

The resulting script is about 1MB of size.

But what we thought lately, that it's overkill to bring whole xslt engine on a client, while it's possible to generate javascript from xslt the same way as he's building java from xquery. This will probably require some runtime but of much lesser size.