Well, I managed to identify the problem. As tvanfosson said, this is because WebKit does not load images. To get around this, I just load both images in an unclicked class
<style> .unclicked { background-image: url('/Portals/_default/images/buttons/checkout-end-disabled.gif'); background-image: url('/Portals/_default/images/buttons/checkout-end.gif'); } .clicked { background-image: url('/Portals/_default/images/buttons/checkout-end-disabled.gif'); } </style>
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