We can develop a spec in a vacuum, but we can't be sure what users actually want from our refrigerator. Maybe a live penguin actually does belong in there, as in the famous children's novel.
Wikipedia has an example of a tag soup https://en.wikipedia.org/wiki/Tag_soup which is probably poster child example of this law:
<p>This is a malformed fragment of <em>HTML.</p></em>
What do we do here? Do we do what browsers have historically done? Or do we throw our hands up in the air when we see malformed tags?
You can perhaps (but my confidence is low) avoid violating the first part of the law if you're a TCP stack implementor, say. There just aren't that many such implementations nowadays and they're generally done by people who know what they're doing. But applying Postel's law to something like programming language implementations, where the "implementors" who would need to obey the first part are all the people writing programs in the language, pretty much guarantees violations of that first part. For any given programming language anyone cares about in practice, the number of people who know the spec for it well enough to only write code that conforms to that spec, even on their first try, is _tiny_ compared to the number of people writing programs in the language.
At that point your only option is to accept that the first part is going to be violated, and badly. Only once you have internalized that understanding can you have a constructive conversation about whether you want to do the second part and to what extent. The answers here will vary, of course.