<!DOCTYPE html>
<meta charset="utf-8">
<title>Centered Background Gradient</title>
<link rel="match" href="background-image-centered-ref.html">
<link rel="help" href="https://www.w3.org/TR/css-backgrounds-3/#the-background-position">
<style>
    #a {
        width: 200px;
        height: 150px;
        border-style: solid;
        border-width: 10px;
        background-position: center center;
        background-image: repeating-radial-gradient(blue, green 20px);
    }
</style>
<body>
    <div id="a"></div>
</body>