HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <title>outline-width: 0.25em</title>
  <style>
    body {
      font-family: Arial, sans-serif;
      margin: 16px;
      color: #0f172a;
    }

    p {
      margin: 0 0 12px;
      max-width: 40rem;
      line-height: 1.5;
    }

    div {
      font-size: 16px;
      outline-width: 0.25em;
      outline-style: solid;
      outline-color: #7c3aed;
      border: 1px solid #cbd5e1;
      padding: 12px;
      max-width: 16rem;
    }
  </style>
</head>
<body>
  <p><strong>outline-width: 0.25em</strong> is 4px when font-size is 16px. Change the font size and the outline scales with it.</p>
  <div>0.25em outline</div>
</body>
</html>

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CSS outline-width Property — 0.25em