<div class="container"> <div class="left"></div> <div class="right"></div> </div> <style> * { padding: 0; margin: 0; } .container { width: 600px; height: 300px; display: flex; } .left { flex: 1 2 500px; background: red; } .right { flex: 2 1 400px; background: blue; } </style>