Opinions vary on how good an idea the robustness principle is. That is why, for example, the XML standard requires a conforming validator to throw an error on invalid XML.

In our modern world, the robustness principle has become an invitation to security bugs, and vendor lock-in. Edge cases snuck through one system on robustness, then trigger unfortunate behavior when they hit a different system. Two systems tried to do something reasonable on an ambiguous case, but did it differently, leading to software that works on one, failing to work on the other.

But, this is robust? I mean it's pretty clearly stating that you are visiting an unsupported URL. It provides direction on what to do about it to the user. It does not crash the browser or the server. In pretty much every dimension this is highly robust.

The robustness principle was a mistake. The attitude it promulgated is a big reason why interoperability is so difficult.

The robustness principle is itself bad manners, in plenty of contexts. If I deliver packages by throwing them at the customer, I really want a customer to tell me "hey, don't throw packages at me!" before I attempt to lob something fragile and breakable, or something heavy at someone fragile and breakable. Otherwise, how am I supposed to learn that I'm doing anything wrong?