<button class="number-minus" type="button" onclick="this.nextElementSibling.stepDown(); this.nextElementSibling.onchange();">-</button>
<input type="number" class="input input--quantity inputQuantity" value="1" min="1" max="999">
<button class="number-plus" type="button" onclick="this.previousElementSibling.stepUp(); this.previousElementSibling.onchange();">+</button>