Forcing form text to be lower-case

Question

How could I force the text in the \"username\" text input to be lower-case regardless of what user types?

Answer 1

@fdiv_bug's answer is good except for the issues mentioned in the comments of their answer. Using the input event instead of the keyup event fixed those issues for me.

HTML:

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Javascript:

element.addEventListener('input',function(){this.value=this.value.toLowerCase()});​