<!DOCTYPE html>
<html lang="en">
    <head>
        <style>
            #back {
                width: 700px;
                height: 500px;
                background-color: green;
            }
            #front {
                width: 500px;
                height: 300px;
                background-color: red;
                border: 50px solid blue;
                padding: 50px;
                -webkit-mask-image: linear-gradient(45deg, black, transparent 100%);
                -webkit-mask-size: 151px 128px;
                -webkit-mask-repeat: space;
                -webkit-mask-origin: border-box;
                -webkit-mask-clip: border-box;
            }
        </style>
    </head>

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