aria-labelledby
to stitch together an external text element with the internal value, this creating a compound label.<span id="el1">Colour</span><button aria-labelledby="el1 el2"><span id="el2">blue</span></button>
<button>Colour: blue<button>
<div role="option">Colour blue</div>
SPACEBAR
will will toggle expanded state of button.ESCAPE
key will collapse button.