The issue is that the client can be bugged. A provider can have a solid client-side encryption solution, but silently switch the client distributed to users of

The issue is that the client can be bugged. A provider can have a solid client-side encryption solution, but silently switch the client distributed to users of interest to an identical version which also phones home with their passwords. It's possible to verify that your code was signed with your provider's certificate, but how do you know it's the right client? How do you know it's the same one that's been peer-reviewed? We'd need a bulletproof way for the browser to check the hash of JavaScript before executing it. Unless you're going to copy/paste and MD5 the source for GMail every time you check it. The tools to be sure of the executable you're running really only exist for desktop software right now.