Postel's law was a precept of the Internet of the 80's and 90's, when due to the primitive software engineering practices at that time, implementations couldn't be tested properly. That lead to many cases of poor interoperability, and it's no longer a good idea: for example, when HTML 5 was designed, they decide to put into the spec how to deal with the frequent errors like mismatched closing tags, etc... because all major implementations were "liberal" in what they accepted, but each in a different way.