The following produces a rectangle with square corners instead of rounded corners: <div style="-webkit-appearance: media-controls-dark-bar-background; border-radius: 25px; width: 200px; height: 50px; will-change: opacity;"></div>
<rdar://problem/31759785>
Dave Hyatt explained on IRC that this is correct behaviour since -webkit-apparance ignores anything to do with margins and decoration in general, since the appearance may provide its own custom setting for this.