jQuery UI seems to be a core module of ResourceLoader.

In T280733#7445872, @Pols12 wrote:

jQuery UI seems to be a core module of ResourceLoader.

It isn't.

This appears to be a faulty gadget? jQUI isn't a standard shipped library; we're trying to delete it, not increase its use. Removing the out-of-scope tags, but this is probably Invalid.

On pages where the jQuery UI styles are loaded, it is loaded coincidentally for unrelated reasons (e.g. there might be a gadget or extension loaded on that page that uses jQuery UI for its own purposes).

MediaWiki has never supported loading jQuery UI styles for templates.

Some wikis used to have a snipper of code in Common.js that automatically looked for the "clickable button" template being used on that page, and if so, loaded the megabyte of JS/CSS of jQuery UI in order to make the button blue. This is of course very costly for such a simple visual effect and to my knowledge no wikis do this anymore.

I would recommend changing or replacing the template with something like https://commons.wikimedia.org/wiki/Template:Clickable_button_3 which uses mw-ui-button styles instead, which are supported by MediaWiki, are very light, are automatically loaded when needed, and look consistent with other parts of MediaWiki.