Login to a Username Hope about operation in case html element is contained.
when usename admin
1. Input Username field value what <h2>admin</h2> then can logged in
2. Input Username field value what <script>admin</script> then return error ERROR: The username field is empty.
It maybe Natural operation of the function wp_strip_all_tags().
It is felt unnatural although I think that such operation is not actually performed.
When a html element is used for a user name, I regard the error that it is Bad username as immediate as it is better to be contrary.