I agree that in general Postel's law is bad practice but I think that it also makes sense under certain contexts. For example, if you have an email input field,

I agree that in general Postel's law is bad practice but I think that it also makes sense under certain contexts. For example, if you have an email input field, it's better to trim spaces from the user input before sending it to the server instead of showing an error that there's spaces needing to be trimmed. In this example the input field is liberal in what it accepts and conservative in what it sends to the server.