HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <title>border-block-color with writing-mode</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 {
      writing-mode: vertical-rl;
      border-block: 4px solid;
      border-block-color: #2563eb #0f766e;
      padding: 12px;
      height: 12rem;
    }
  </style>
</head>
<body>
  <p>The colors still mean block-start and block-end. In <strong>vertical-rl</strong> those edges are right and left.</p>
  <div>Colors follow writing mode</div>
</body>
</html>

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CSS border-block-color Property — writing-mode