It really depends on what you're trying to do. Amongst other things, since you can't ever know the Javascript runtime in the browser hasn't been tampered with,

It really depends on what you're trying to do. Amongst other things, since you can't ever know the Javascript runtime in the browser hasn't been tampered with, and that the functions you're calling from even correct crypto code haven't been overridden, the system will never actually be safe to trust.