1. :default CSS pseudo-class

    The `:default` pseudo-class matches checkboxes and radio buttons which are checked by default, `<option>`s with the `selected` attribute, and the default submit button (if any) of a form.

  2. htmloptionelement api: defaultselected

  3. htmloptionelement api: selected

  4. htmloptionscollection api: selectedindex

  5. htmlselectelement api: selectedoptions

  6. html element: option: selected