This is really a UX question, not design per se.
https://gerrit.wikimedia.org/r/#/c/104926/ (an under-review change) adds
a warning if you choose a username that needs to be
reformatted/canonicalized. For example, "my username" is changed to "My
username".
It displays the warning, puts the new username ("My username") in the
box, then you have to retype your password (twice), then press enter to
confirm.
The main potential benefit, as I see it, is that if you don't like "My
username" (the reformatted version), you can choose a different username
entirely.
User experience feedback would be welcome; you can post on the change or
here.
Matt Flaschen