The below example is just to set the height and width of <html> and <body> to 100%, overflow to hidden and use percentages for left, top, width and height of elements:
<!DOCTYPE html>
<html>
<head>
<meta charset=UTF-8>
<title>Proportional resizing</title>
<style type="text/css">
* {
margin: 0;
padding: 0;
}
html, body {
height: 100%;
overflow: hidden;
}
div {
position: absolute;
left: 30%;
top: 20%;
width: 40%;
height: 30%;
background-color: #ddd;
}
</style>
</head>
<body>
<div>divthing</div>
</body>
</html>
Hope this helps you