Haven't read through the WebCrypto standard, but it's hard to create a real barrier between a server process and a client process on the web. In essence, any good crypto UI would have to happen outside of the client process, so that the server cannot intercept your input. (This is my objection to well-meaning companies that say "Hey, we can't even read your stuff, it's encrypted on the client" because, if you have auto-update, or if you ccan inject code into the client, that privacy guarantee can be revoked at any time.)