We don’t want to invalidate the input if the user removes all text. They may need a moment to think, but the invalidated state sets off an unnecessary alarm.

Just let the user fill in some fields, submit it to the server and if there are any errors notify them and let the user start over again. Is that a good approach? The answer is no, you don't want users to get frustrated waiting for a server round trip to get some form validation result.

So when we ask users to answer questions that deal with the future, we have to keep in mind the context in which they’re answering. They can tell us about a feature they think will make their lives better, but user val-idation will always be necessary to make sure that past user’s beliefs about future user are accurate.