stepUp и stepDown не работают в Edge и IE.
<span class="minusIcon" onclick="document.getElementById('quantity').stepDown(1)"">-</span>
<input type="number" name="quantity" min="1" disabled="disabled" max="10" id="quantity" class="quantity" value="1" />
<span class="plusIcon" onclick="document.getElementById('quantity').stepUp(1)">+</span>
JS Fiddle: https://jsfiddle.net/ngcputf5/