<!DOCTYPE html>
<html>
<head>
    <style>
        .container {
            margin: 50px;
            height: 300px;
            width: 300px;
            border: 1px solid black;
        }
        
        .transformed {
            width: 100%;
            height: 100%;
            background-color: blue;
            transform: rotateY(45deg);
            transform-origin: top left;
        }
    </style>
</head>
<body>
    <div class="container">
        <div class="transformed"></div>
    </div>
</body>
</html>