Non-standard
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.
The ::-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.
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https://developer.mozilla.org/en-US/docs/Web/CSS/::-ms-expand