From the title of the issue: “Form-associated custom elements”. They are talking about the mechanism by which a custom element can have its value included when you submit a form. Here’s the spec.:

https://html.spec.whatwg.org/multipage/custom-elements.html#...

https://html.spec.whatwg.org/multipage/form-control-infrastr...

https://html.spec.whatwg.org/multipage/form-control-infrastr...