Postel's law

Postel's law or the Robustness principle states "an implementation should be conservative in its sending behavior, and liberal in its receiving behavior", sometimes restated as "Be liberal in what you accept, and conservative in what you send".

In the context of the IndieWeb, implementations should strive to publish well-formatted, understandable documents (someone may View Source and use your page as an example), but should strive to accept a wide variety of inputs with sane fallback behavior.