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NIST proposes barring some of the most nonsensical password rules
(arstechnica.com)
This is a most excellent place for technology news and articles.
I'm with you, despite seeing lemmings downvote the heck out of your comment 😢
The reason, and specifically for whitespace at the beginning or end of a password, is that a lot of users copy-paste their passwords into the form, and for various reasons, whitespace can get pasted in, causing an invalid match. No bueno.
Source: I'm a web developer who has seen this enough times that we had to implement a whitespace-trim validation for both setting & entering passwords.
Trimming whitespace from the start and end of a password is fine but you absolutely should not remove whitespace from the middle of a password.