<input type="text" class="quantity"
  name="Quantity"
  id="Quantity"
  onkeydown='return (window.event.keyCode >= 48 && window.event.keyCode <= 57) || window.event.keyCode == 8 || window.event.keyCode == 46'
  required />

<input type="text" class="quantity"
  name="Quantity"
  id="Quantity"
  onClick="this.select();"
  onkeydown='return (window.event.keyCode >= 48 && window.event.keyCode <= 57) || window.event.keyCode == 8 || window.event.keyCode == 46'
  required />