web-animations/responsive/backgroundPosition.html

<!DOCTYPE html>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

<div id='container'>
<div id='element'></div>
</div>

<script>
var element = document.getElementById('element');
var container = document.getElementById('container');

test(function() {
    element.style.fontSize = '10px';
    var player = element.animate([
        {backgroundPosition: '10% 1em'},
        {backgroundPosition: '20% 5em'}],
        10);
    player.pause();
    player.currentTime = 5;
    element.style.fontSize = '20px';
    assert_equals(getComputedStyle(element).backgroundPosition, '15% 60px');
}, 'Background position responsive to style changes');

</script>