HTML Code

<!DOCTYPE html>
<html>
<head>
  <title>CSS border-block-start-color Vertical Writing</title>
  <style>
    body {
      font-family: sans-serif;
      padding: 1.5rem;
      background: #f8fafc;
    }
    .vertical-panel {
      writing-mode: vertical-rl;
      border-block-start-style: solid;
      border-block-start-width: 3px;
      border-block-start-color: #7c3aed;
      padding: 1rem;
      background: #faf5ff;
      display: inline-block;
      font-weight: 600;
    }
  </style>
</head>
<body>
  <h1>Vertical writing mode</h1>
  <div class="vertical-panel">Vertical block-start color</div>
  <p>The block-start color follows the writing mode.</p>
</body>
</html>

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CSS border-block-start-color Property - Vertical Writing Mode