[kamjam]

Fair point, and a browser extension that would detect changes in he script would be easy enough to build. The point is, for the vast majority of users, freezing certs and such is not common (I didn't know you could do that for example) and they would rely on the site to specify the cert to use.

[lemonade]

This is called certificate pinning (a.k.a. Trust On First Use or TOFU). If you use Firefox, just install Certificate Patrol.

If you are interested in the rationale, and how you can use technologies like DANE to make it even better, read the paper by Gabor Toth and Tjebbe Vlieg (http://staff.science.uva.nl/~delaat/rp/2012-2013/p56/report....).