@charset "utf-8";
.learn {
}
.hidden { display: none; }
.unhidden { display: block; }

If you want to use visibility, then change these classes to:
.hidden { visibility: hidden; }
.unhidden { visibility: visible; }

Then add the hidden class to your DIV so that it starts out as hidden on the page:
<div id="learnHTML" class="hidden">