Could you please elaborate, so that more people do not fall into this intellectual trap?
Basically, the server you're talking to, as well as any resources on that page, can undermine your javascript primitives and render your crypto useless (or just backdoor it).
If you trust the server to not backdoor your crypto... you can just trust the server to _do_ the crypto in the first place.
There is an effort underway to build better crypto APIs into browsers, but I'll bet you a bitcoin that it's super easy to fuck up the implementation of and most end up being insecure, and/or nobody ends up using it after all.
Google controls the key/cert that allows for Chrome extension updating...