For the lazy like me: robustness principle, aka Postel's law
https://en.wikipedia.org/wiki/Robustness_principle
Thank you for the reference. I learned something today!
> A flaw can become entrenched as a de facto standard. Any implementation of the protocol is required to replicate the aberrant behavior, or it is not interoperable. This is both a consequence of applying Postel's advice, and a product of a natural reluctance to avoid fatal error conditions.
[0] https://tools.ietf.org/html/draft-iab-protocol-maintenance-0...