<style>

#button {
    -webkit-appearance: -apple-pay-button;
}

.white {
    -apple-pay-button-style: white;
}

.black {
    -apple-pay-button-style: black;
}

</style>
<script>

if (window.testRunner)
    testRunner.waitUntilDone();

function load()
{
    setTimeout(() => {
        button.className = "white";

        if (window.testRunner)
            testRunner.notifyDone();
    }, 0);
}

</script>
<body onload="load()">
<button id="button" class="black"></button>