A form-associated custom element exposes itself to ARIA via `ElementInternals` (e.g. `internals.role = 'slider'`, `internals.ariaValueNow = '5'`). An author then also writes `aria-valuenow="9"` as a content attribute on the element in the light DOM. Which value wins in the accessibility tree, and what is the design intent?
this.internals_.role = 'slider';
this.internals_.ariaValueNow = '5';
// author markup: <my-slider aria-valuenow="9"></my-slider>