Since no one else has explicitly brought this up: https://tonyarcieri.com/whats-wrong-with-webcrypto (also see Matasano's "JavaScript Cryptography Considered Harmful", etc.). There is nothing that a service like this will protect you from that you don't already get while using TLS in just about every insecure/non-E2EE chat service. If you're looking for actually secure communication that runs as a Web app, my startup…
> the scheme to accomplish this was rigorously vetted in a 12-day audit. Vetted by whom? It would help build trust if there was a public report on the strength of your implementation available somewhere.
(That link contains a page of additional context to explain the findings, e.g. some were pre-production.)