так пробуй
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<script>
var i = -975;
function smoothCh() {
var idCase = document.getElementById('case');
if (i == 0) clearTimeout(timer);
else {
idCase.style.backgroundPosition = ++i + 'px 0px';
var timer = setTimeout(smoothCh, 20);
}
}
document.addEventListener('DOMContentLoaded', smoothCh)
</script>
</head>
<body>
<div id="case"></div>
</body>
</html>