Be conservative in what you send, be liberal in what you accept
The principle is also known as Postel’s law, after Jon Postel, who used the wording in an early specification of TCP.
“It takes two to miscommunicate.” A great listener, or a skilled speaker, can resolve a lot of conflicts.
Postel’s Law is the principle the Internet is based on. Not because Jon Postel was such a great salesperson and talked everyone into it, but because that is the only winning evolutionary strategy when internets are competing.
Structure from Structurelessness
Allow all, bless some is an important general principle for building evolvable systems. Allowing all means new use-cases can evolve from the bottom-up, while blessed fields provide a stable API which multiple clients can count on
This is how protocols evolve