chromium/third_party/blink/web_tests/fast/forms/number/number-spinbutton-increment-decrement-state-expected.html

<input id="num1" type="number" value="0">
<div>input number value: 1</div>
<script>
    var num1 = document.getElementById("num1");
    var verticalOffsetToUpButton = 5;
    var upButtonX = num1.offsetLeft + num1.offsetWidth - 5;
    var upButtonY = num1.offsetTop + verticalOffsetToUpButton;
    eventSender.mouseMoveTo(upButtonX, upButtonY);
    // Keep the mouse down on the up button to give it "active" styling.
    eventSender.mouseDown();
</script>