The only guarantee TOFU gives, is "this is the same key as you were communicating with last time" - which is simple enough that people can reason about it, and yet strong enough that it significantly boosts security.
TOFU is an excellent baseline, people who need more can augment it by verifying keys out of band, pinning keys, etc.
Simplicity matters! For code and UX.
Bad UX = mistakes = insecurity.
The security of WKD piggy-backs off TLS and the CA-based trust model of the web.
The WKD promise is "the website at foo.com claims this is the key for email@example.com". That's a good baseline, assuming user is on friendly terms with his domain and website admin.
WKD is great for institutions and organizations, maybe less awesome for Joe Random Activist.
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