gecko-dev/image/test/reftest/colordepth.html

<script>
/*
 * Almost all of the image decoding reftests require the display to be in
 * 24-bit color mode, or else the rendered images will have subtle color
 * variations and will fail.
 * The Windows test boxes used to have a tendency to flip to 16-bit color mode,
 * so this test will explicitly check the color depth to make it more obvious
 * when the requirement is not met. (See bug 414720.)
 * 24-bit is only a minimum, allow higher values too. (See bug 458847.)
 */

var colorDepth = window.screen.colorDepth;

if (colorDepth < 24)
    document.write("ERROR: color depth is only " + colorDepth + ".");
</script>