It's from an era when interoperability in a then-very-small universe was very important to moving the Internet forward. Back then there were no spammers, and very few bad actors. It's possible that the "Postel law" helped for a while -- hard to say for sure though.

It makes sense for a lot of situations purely out of practicality. If a good portion of your customers use tools that break spec, your choices are to accept the brokenness or lose the customer.

Take browsers, for example. A lot of people complain that browsers try to work around broken certificate setups on servers, but if they didn’t, users would just switch to browsers that did.

You're not wrong, but sometimes you have to be pragmatic. I've done a lot of work building interfaces to acquire clinical data from healthcare provider organizations for care quality improvement. There are clear industry standards from HL7 (V2 Messaging, CDA, FHIR) but many provider organizations don't implement them correctly, so we had to accept a lot of junk and just find a way to deal with it. Many of those provider organizations lack IT resources and are at the mercy of their vendors or consultants, so trying to hold them to strict standards results in an impasse.

Idk it makes sense for UX sometimes. Really it’s a case by case basis and I don’t think you should be adhering to any law for this.