1. Dialog element

    Method of easily creating custom dialog boxes to display to the user with modal or non-modal options. Also includes a `::backdrop` pseudo-element for behind the element.

  2. element api: pseudo: `type` parameter: `"::backdrop"` value

  3. css selector: `::backdrop`

  4. css selector: `::backdrop`: support on `dialog` elements

  5. css selector: `::backdrop`: fullscreen support

  6. css selector: `::backdrop`: backdrop elements inherit their values from its originating element

  7. css selector: `::backdrop`: support on popovers