HTML Code

<!DOCTYPE html>
<html>
<head>
  <title>CSS :optional Form Example</title>
  <style>
    body {
      font-family: system-ui, sans-serif;
      padding: 1.5rem;
      max-width: 24rem;
    }

    label {
      display: block;
      margin-bottom: 0.25rem;
      font-size: 0.875rem;
      font-weight: 600;
      color: #334155;
    }

    input, textarea {
      display: block;
      width: 100%;
      margin-bottom: 1rem;
      padding: 0.55rem 0.75rem;
      font-size: 0.9375rem;
      border: 1px solid #cbd5e1;
      border-radius: 0.4rem;
      font-family: inherit;
      box-sizing: border-box;
    }

    textarea {
      min-height: 4rem;
      resize: vertical;
    }

    form input:required,
    form select:required {
      border-left: 4px solid #dc2626;
      padding-left: 0.65rem;
    }

    form input:optional,
    form select:optional,
    form textarea:optional {
      border-left: 4px solid #2563eb;
      padding-left: 0.65rem;
      opacity: 0.95;
    }
  </style>
</head>
<body>
  <h1>Sign Up</h1>
  <form>
    <label for="user">Username *</label>
    <input type="text" id="user" required placeholder="Username *">

    <label for="company">Company (optional)</label>
    <input type="text" id="company" placeholder="Company (optional)">

    <label for="bio">Bio (optional)</label>
    <textarea id="bio" placeholder="Bio (optional)"></textarea>

    <button type="submit">Create account</button>
  </form>
</body>
</html>

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CSS :optional - Registration Form