coba.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      .kotak {
        width: 200px;
        height: 200px;
        background-color: black;
        z-index: 1;
      }

      .kotak::after {
        content: '';
        display: block;
        width: 200px;
        height: 200px;
        background-color: darkred;
        opacity: 0.8;
      }
    </style>
  </head>
  <body>
    <div class="kotak"></div>
  </body>
</html>