HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <title>#a, #b</title>
  <style>
    body {
      margin: 16px;
      font-family: system-ui, sans-serif;
      color: #0f172a;
    }

    #hero, #sidebar {
      background: #eff6ff;
      border: 1px solid #93c5fd;
      padding: 0.75rem;
      margin-bottom: 0.5rem;
      border-radius: 8px;
    }
  </style>
</head>
<body>
  <div id="hero">#hero</div>
  <div id="sidebar">#sidebar</div>
  <div>No id</div>
</body>
</html>

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CSS ID Selector — grouped ids