<html>
<head>
<script>
function isElementInViewport(el) {
var top = el.offsetTop;
var left = el.offsetLeft;
var width = el.offsetWidth;
var height = el.offsetHeight;
while (el.offsetParent) {
el = el.offsetParent;
top += el.offsetTop;
left += el.offsetLeft;
}
return (
top < (window.pageYOffset + window.innerHeight) &&
left < (window.pageXOffset + window.innerWidth) &&
(top + height) > window.pageYOffset &&
(left + width) > window.pageXOffset
);
}
</script>
</head>
<body>
<div style="height: 180%; position: relative;">
<button style="position: absolute;">Top</button>
<button style="position: absolute; top: 50%;">Center</button>
<button style="position: absolute; bottom: 0;">Bottom</button>
</div>
</body>
</html>