The `<selectedoption>` element is part of the customizable `<select>`
proposal: whatwg#9799

It allows authors to declaratively clone the contents of the currently
selected `<option>` of a `<select>` and style it independently for use
in a base appearance `<select>`'s button.

The timing of cloning has been discussed here:
whatwg#10520

The selectedoption element has been discussed generally here:
w3c/csswg-drafts#10242