If the revocation data is available from "everywhere", and the browser sees that it hasn't been able to get it recently from "anywhere", that's a suitably alarming situation that the user should get a blocking warning, as with any other self-signed/untrusted-CA certificate.

In the usual browsing situation, it also doesn't have to be a blocking verification before displaying the first bytes from a site (requiring subsecond checking). It could load the page but continue verification in the background, only becoming a blocking warning some number of seconds later, when the user enters form data or attempts other navigation.

In fact, even minutes into a session, the information that either – (a) it took a while but we now see that this certificate is definitely bad; or (b) you are currently so cut-off from the real "Internet" that suspicion towards all sites is justified – is useful.

That is, even a "delayed hard fail" would still be useful.

The alternative is that people chat, transact, etc. indefinitely at the mercy of attackers, because the browser isn't even trying to acquire good data via censor-resistant channels.

You have to keep the online revocation service up all the time. People will DDoS just for the lulz it causes when it being broken breaks everyone's browser.

While sitting in a meeting I realized there is one situation that online revocation helps: someone steals bank.com's DNS record. They can't necessarily MITM anyone with that.

But, TACK is probably a better countermeasure there.

DDoSing DNS would break everyone's browser for lulz too, but somehow the net manages to chug along. And if/when it happens, at least people know they're under attack and direct attention to fixing things.

A service with a clear single point to DDoS is obviously the wrong choice.

What if services all over the net could tell you the latest (<5 minutes old) root/summary-hash of the shared dataset? And, there are many places to either update yourself to that version, or receive a trustworthy proof a certificate is/isn't in that dataset?

If they're all blocked, an alarm should go off, letting the user know it's dangerous to proceed. The failure shouldn't be secret, with the confidence-inspiring lock icon still appearing, for indefinitely long periods.