At some point recently, Special:AbuseFilter/test started requiring a value for "Changes made to page". I mostly use this tool to check a filter pattern against a specific user, without any value for the page. If I use the browser's developer tools to remove the "required" attribute, I see that it works as it did before.

I believe in actuality either "Changes made by user" or "Changes made to page" is required, so you wouldn't be able to do this with HTML attributes alone.