Perhaps... a sandboxed webmail[0] app capable of sending and receiving encrypted (or cryptographically signed) emails without being vulnerable to client-side at

Perhaps... a sandboxed webmail[0] app capable of sending and receiving encrypted (or cryptographically signed) emails without being vulnerable to client-side attacks stealing your private key(s) or the cleartext? There may be similar use-cases for tamper-proof (or at least tamper-evident) browser-based games. [0] I'm specifying a webmail client in the browser rather than a desktop email client.