Browsers are increasingly willing to say no to users over HTTPS issues
One of the quiet sea changes that underpins a significant increase in the security of the modern HTTPS web is that browsers are increasingly willing to say no to users. I happen to think that this is a big change, but it's one that didn't really strike me until recently.
There was a time when the fundamental imperative of browsers was that if the user insisted enough, they could go ahead with operations that the browser was pretty sure were a bad idea; attempts to change this back in the days were met by strong pushback. The inevitable result of those decisions was that attackers who wanted to MITM people's HTTPS connections to places like Facebook could often just present a self-signed certificate generated by their MITM interceptor system and have most people accept it. When attackers couldn't do that, they could often force downgrades to unencrypted HTTP (or just stop upgrades from an initial HTTP connection to a HTTPS one); again, these mostly got accepted. People wrote impassioned security advice that boiled down to 'please don't do that' and tweaked and overhauled security warning UIs, but all of it was ultimately somewhat futile because most users just didn't care. They wanted their Facebook, thanks, and they didn't really care (or even read) beyond that.
(There are any number of rational reasons for this, including the often very high rate of false positives in security alerts.)
Over the past few years that has changed. Yes, most of the changes are opt-in on the part of websites, using things like HSTS and HPKP, but the really big sea changes is browsers mostly do not let users override the website settings. Instead, browsers are now willing to hard-fail connections because of HSTS or HPKP settings even if this angers users because they can't get to Facebook or wherever. Yes, browsers have a defense in that the site told them to do this, but in the past I'm not sure this would have cut enough ice to be accepted by browser developers.
(In the process browsers are now willing to let sites commit HSTS or HPKP suicide, with very little chance to recover from eg key loss or inability to offer HTTPS for a while for some reason.)
Obviously related to this is the increasing willingness of browsers to refuse SSL ciphers and so on that are now considered too weak, again pretty much without user overrides. Given that browsers used to accept pretty much any old SSL crap in the name of backwards compatibility, this is itself a welcome change.
(Despite my past views, I think that browsers are making the right overall choice here even if it's probably going to cause me heartburn sooner or later. I previously threw other people under the HTTPS bus in the name of the greater good, so it's only fair that I get thrown under it too sooner or later, and it behooves me to take it with good grace.)