Short answer, client.js is run on the client only.
Longer answer...
Everything in wm_server is run on the server, everything in wm_client is run in the clients browser. If you check out my previous post on MXP, you'll see how I've implemented it with a whole lot of regex's. This includes stripping out things like Mobs/Items/Prompts/Mini-Maps and putting converting them into HTML "stuff". It's not pretty (yet) but once you extract the data you can do anything you want with it.
Depending on your implementation, just watch that a player can't screw with other people by sniffing out some of your MXP code and then "SAY"ing it or otherwise getting the mud to output it.