Consider adding a headingoffset attribute #5033
Description
See the suggestion by @muan at #3499 (comment):
<h1>GitHub</h1> <h2>jsdom/jsdom</h2> <div> <article headinglevelstart="3"> <h1>jsdom</h1> <h2>Basic usage</h2> <h2>Customizing jsdom</h2> <h3>Simple options</h3> ... </article> </div>
cc @whatwg/a11y