WooCommerce uses HTML5 number fields for shop quantities, because they restrict the characters you can enter, and Safari on iPad/iPhone conveniently shows the number keyboard. Webkit and Opera/Presto add spinners (up/down arrows) to HTML5 number fields. WooCommerce also adds +/- buttons surrounding qty fields, because IE and Firefox don't add spinners. WooCommerce then uses CSS to hide the spinners on Webkit:
The Opera browser persistently refuses to honour "return false" in an onclick handler, and is quite possibly the only web browser that does this. Because I don't usually add event handlers inline any more, I haven't noticed this defect until now, but it just bit me today.