main {
  max-width: 800px;
  margin: 0 auto;
  form {
    max-width: 50%;
    margin: 0 auto;
    input:not([type="submit"]):not([type="checkbox"]) {
      width: 100%;
    }
    input[type="submit"] {
      display: block;
      margin: 4px auto;
    }
  }
}