Crazy, I have been doing that for over 20 years, first time having a problem with that. Would be good to have a message about that on a submit attempt.
If a trailing / (slash) character is present in the start tag of an HTML element, HTML parsers ignore that slash character. This is important to remember when an element such as <script> or <ul> does require a closing tag. In this case, adding a trailing slash in the start tag does not close the element.