Hi

When I add an embed to the page the editor adds "Default preview for : Embed". If I remove this text and switch between html and normal view the text re-appears.  Tested with chrome and IE11 on your demo page.

To recreate:

1: Goto http://www.richtexteditor.com/demo/

2: Paste <embed id="wiggle"></embed> into html view.

3: Switch to normal view and submit. 

Can you stop this from appearing please.