Spacing etc. is quite broken. Also will let other tasks like disable-and-check checkboxes when the enable-all button is checked be done much more easily.
Not sure what this task is supposed to cover, but I think it has been mostly done with rEBET14379dd886f6: NewHTMLCheckField: Use pretty field for OOUIHTMLForm too as part of the work on T117781. There are some CSS classes that are prefixed with mw-ui-, but they are all custom and the prefix is harmless, we are not loading any of normal MW UI styles.