transparent_test.html

<!DOCTYPE html>
<html>
    <head>
        <style>
            html, body {
                background-color: transparent;
                height: 100%;
                width: 100%;
            }
            #untransparent {
                background-color: rgba(255, 255, 255, 0.5);
                height: 100%;
                width: 50%;
            }
        </style>
    </head>
    <body>
        <div id="untransparent"></div>
    </body>
</html>