HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <title>CSS px Example</title>
  <style>
    body {
      margin: 0;
      padding: 1.5rem;
      font-family: system-ui, sans-serif;
      background: #f8fafc;
    }

    .box {
      width: 300px;
      height: 80px;
      background: #2563eb;
      color: #fff;
      display: flex;
      align-items: center;
      justify-content: center;
      border-radius: 0.35rem;
      font-size: 0.875rem;
    }
  </style>
</head>
<body>
  <h1 style="font-size:1.125rem;color:#1e293b;">300px wide box</h1>
  <div class="box">width: 300px (absolute)</div>
</body>
</html>

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Absolute Length — px Units