<head>
  <meta charset="UTF-8" />
  <title>Masking</title>
  <style>
    .block {
      height: 100vh;
    }

    .mask-container {
      width: 300px;
      height: 300px;
      background: yellow;
      transform: translateX(0);
    }

    .mask {
      position: fixed;
      left: 0;
      right: 0;
      top: 0;
      bottom: 0;
      background: rgba(0, 0, 0, 0.3);
    }
  </style>
</head>

<body>
  <div class="block"></div>
  <div class="mask-container">
    <div class="mask">Mask</div>
  </div>
</body>
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