<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<style>
    body {
        margin: 0;
        padding: 0;
    }
    iframe {
        border: none;
        background-color: green;
        width: 100vw;
        height: 100vh;
        box-sizing: border-box;
        display: block;
    }
</style>
</head>
<body>
    <iframe srcdoc="<button onclick='document.documentElement.requestFullscreen();'>Go fullscreen</button><p>This test passes if background is fully green with no red.</p>"></iframe>
</body>
</html>
