There is a problem with CKEditor when the user inserts empty tags:
(in HTML source mode)
<a href="fds" class="doIt"></a>
When he presses on the source and then back again, the tag 'A' is gone. I made a little research, and I found that this happens since there is a very complicated function in ckeditor.js that checks whether there is a value inside the element. If there's none the CKEditor drops the tag.