I would guess CKEditor is changing/reformatting the HTML for you. If you really want to allow HTML input, you should normalize it as that is the only way to decide <p>OHAI!</p> is equal to <p>\nOHAI!\n</p>, which it is, as far as HTML is concerned, and unless you are doing wonky stuff with CSS/JS to create significance in otherwise meaningless white space. This is not quite as easy as it might sound, but it's doable if that's really what you're after.

Personally, I would choose client side / JS tracking of change instead, and not submit unless there is a change, then let the backend always update. It's "hackabe" in the sense a user could turn off JS or POST directly with any number of tools but if you have the permission and data checks you should have anyway on the backend the worst the "hack" could do is waste a couple tenths of a second of clock time. The approach would be cheaper than creating, normalizing, and validating 4 or more HTML document fragments on every submit.

Update: another approach. Apply CKEditor to the hidden field so it gets the same processing as the visible one by instead of using hidden use something like <textarea style="display:none" ...>.

Update: s/of/off/; s/seconds/second/; :P