jQuery $ no está definido
<!--If that error occurs, you need to include Jquery in the HTML-->
<script src="https://code.jquery.com/jquery-3.3.1.min.js"></script>
Dr. Hippo
<!--If that error occurs, you need to include Jquery in the HTML-->
<script src="https://code.jquery.com/jquery-3.3.1.min.js"></script><script src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js" type="text/javascript"></script>