<!DOCTYPE html>
<style>
    div#div1 {
        background: blue;
        width: 200px;
        height: 200px;
    }
    div#div2 {
        background: red;
        width: 2000px;
        height: 2000px;
    }
</style>
<body>
    <div id="div1">will go fullscreen</div>
    <div id="div2">will not go fullscreen</div>
</body>