Probably something new would have to be invented as there is no mechanism to send messages to the browser in plain html. Something equivalent of a form element but without the action attribute. It would be nice to have something like (spitballing)

  <some-new-form>
    <input type="hidden" value="fullscreen" />
    <input type="submit" value="Go fullscreen" />
  </some-new-form>

> Probably something new would have to be invented as there is no mechanism to send messages to the browser in plain html.

Web components. At least their usage is plain HTML and could do what you want internally with the JS API.

Oh please no, we barely got rid of the target attribute /: