input:invalid {
  border: 1px solid red;
}

.codebox {
  border: 1px solid #ccc;
  background-color: #f8f8f8;
  padding: 10px;
  margin-bottom: 10px;
}

.list {
  font-family: sans-serif;
  list-style-position: inside;
  padding-left: 0;
}