Hi, it might be that you putting a class on an element that does not by default support class as valid attribute.
Check this out, if your HTML markup does not contain the classes/elements you added: http://cjsharp.com/blog/2013/04/15/how-to-add-valid-elements-like-iframes-to-tinymce-in-episerver-7/, but as Linus wrote, it would be better to get more details :)
I'm trying to insert HTML content to a xhtmlstring property type which has bootstrap classes.
The bootstrap js library does not affect the html inserted by the EPpServer. I assume because the episerver loads the HTML after the dom is created, hence the problem.
Any idea how to solve this?