<!DOCTYPE html>
<html lang="en">
    <head>
        <style>
            #back {
                width: 800px;
                height: 400px;
                background-color: yellow;
            }
            #front {
                width: 600px;
                height: 200px;
                background-color: green;
                border: 50px solid blue;
                padding: 50px;
                -webkit-mask-image: linear-gradient(45deg, white, black);
                mask-source-type: luminance;
                -webkit-mask-size: 200px auto;
            }
        </style>
    </head>

    <body>
        <div id="back">
            <div id="front" />
        </div>
            </body>
</html>