HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <title>inset-block-start 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;
    }

    .stage {
      position: relative;
      height: 160px;
      border: 1px dashed #94a3b8;
      writing-mode: vertical-rl;
    }

    .box {
      position: absolute;
      inset-inline-start: 12px;
      inset-block-start: 20px;
      background: #2563eb;
      color: #fff;
      padding: 8px;
    }
  </style>
</head>
<body>
  <p>The value still means block-start. In <strong>vertical-rl</strong> that is not the top edge.</p>
  <div class="stage">
    <div class="box">Follows block-start</div>
  </div>
</body>
</html>

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CSS inset-block-start Property — writing-mode