This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
::-ms-expand CSS pseudo-element represents the button within a
<select> that opens or closes the drop-down menu that displays the
<option>s. It often normally looks like a triangle that points downward.
Not part of any specification.
Microsoft has a description on MSDN.