I'm still trying to understand how your key/seed is stored securely in browser? I guess maybe I missed the boat on this... I've tried asking, but never any responses.
Check out that sexy progress bar on nostr:npub1mutnyacc9uc4t5mmxvpprwsauj5p2qxq95v4a9j0jxl8wnkfvuyque23vg. 
Discussion
I don’t have any particular knowledge of Mutiny’s design, but I assume that all of the pros and cons of client-side browser data storage apply. It’s stored just as any data is stored. Keeping in mind that this is a spending wallet, so the risk profile is such. And the code is all there on GitHub to examine.