WebUI: omit closing on HTML void elements

https://developer.mozilla.org/en-US/docs/Glossary/Void_element#self-closing_tags :
>Self-closing tags (<tag />) do not exist in HTML.

src/webui/www/.htmlvalidate.json

@@ -6,7 +6,6 @@
     "long-title": "off",
     "no-inline-style": "off",
     "prefer-button": "off",
-    "void-style": "off",
     "wcag/h63": "off",
     "wcag/h71": "off"
   }