If you're interested in special logic for specific websites, the WebKit Quirks.cpp [0] is well worth a look over.

0: https://github.com/WebKit/WebKit/blob/main/Source/WebCore/pa...

Uhm, there's about:compat, edge://compat/, WebKit Quirks (https://trac.webkit.org/browser/webkit/trunk/Source/WebCore/...), and Chrome (https://source.chromium.org/chromium/chromium/src/+/main:con... and https://source.chromium.org/chromium/chromium/src/+/main:com...).

Funny enough, I also helped write the about:compat UI and work on the related site interventions.

Site compatibility is a surprisingly broad topic that doesn't tend to fit neatly into our ideal vision of the web.

Yes, though in this case it's for sites relying on Facebook's login authenticator, not really Facebook itself.

SmartBlock also contains special logic for Google Analytics, Google Publisher Tags, and other trackers that cause sites to break when blocked. It's not meant for their benefit, but the user's.

In fact all browsers have special web-compatibility interventions for sites which don't work correctly in them. Even Chrome. And sometimes even for major sites and services, especially when it comes to anti-tracking features.

They have had a special logic AGAINST Facebook for a long time now, via Facebook containers

That's a separate extension, with Facebook clearly in its name.