Postel's Law works when everyone follows it, but it's unstable. At some point somebody will be liberal in what he sends and conservative in what he accepts and everyone else will end up working around his bugs forever. This can even lead to a ratcheting effect over time as people use less and less of the protocol to avoid previous bugs.

This leads us to Ruby's Corollary[1]: Be exact in what you send and liberal in what you accept. Since programmers will neither read the spec nor run the test suite, you should build real implementations that exercise every facet of the spec (e.g. randomize anything that doesn't matter). The only way to interoperate with such implementations is to do things more or less correctly.

[1] After Sam Ruby, who inserts test cases into his blog to encourage browsers and feed readers to work better.