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Spaces, unfortunately, can be part of a MySQL server username.

It is very counter-intuitive, it is extremely rare, but since I got one report about someone not being able to connect to such a server with this kind of a username I have a user on my local MySQL server with the username space  (since you can't really see it unless you view the source of the HTML reply, it is space followed by a space character). Yes, you can use it just fine to log into the MySQL server. It's maddening, but it's real.

Since leading and trailing spaces are legitimate username characters I am unfortunately unable to trim the username provided as it would break legitimate connections with this kind of uncommon username.

I wish I was making this up. My life would be far simpler if I could just yeet invisible leading and trailing characters from username fields. As you have intuited, it's not an uncommon problem.

There is no way to format an HTML input field to make spaces obvious. You cannot apply a different background to the input text than the rest of the field's background, for example. Nor can you tell a browser to render visible whitespace like Word does for example.

You can, however, spot that you have an extra space because the caret is not adjacent to the last character you input but off to the right. This is the tell-tale sign that you have entered an errant trailing space.