[css-forms-1] Vertically centering form control content #14319

The UA stylesheet currently applies block-axis centering in a few places: options, radio buttons, checkboxes, and buttons. But actually, when an input or select is sized larger than the text and icons it contains, its contents should also be centered.

We should apply centering systematically to all single-line form controls:

input, select, option {
  align-content: center;
  align-items: center;
}

I think the main question I have is how much of this needs to be restricted to appearance: base.