HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>CSS box-sizing Form Input</title>
  <style>
    body {
      font-family: sans-serif;
      padding: 1.5rem;
      background: #f8fafc;
    }

    form {
      max-width: 16rem;
    }

    label {
      display: block;
      margin-bottom: 0.35rem;
      font-size: 0.875rem;
    }

    input[type="text"] {
      box-sizing: border-box;
      width: 100%;
      padding: 0.65rem 0.75rem;
      border: 1px solid #cbd5e1;
      border-radius: 0.5rem;
    }
  </style>
</head>
<body>
  <h1>Full-width input</h1>
  <form action="#" onsubmit="return false">
    <label for="email">Email</label>
    <input type="text" id="email" placeholder="you@example.com">
  </form>
</body>
</html>

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CSS box-sizing Property - Full-Width Form Input