I have implemented two versions of previews for our editor:
The intention of this preview is to help during complicated math equations. Every change in the text is displayed immediately on the preview nearly with the layout you will get if the post is submitted.Unfortunately it was not possible to synchronize the scrollbars of the editor and of this preview, mainly also because the size of math expressions is not the same in the editor and in the resulting post.
This type of preview is dedicated to give a whole picture of a post at once. Every time you check the box of this preview, a snapshot of the actual content of the editor is taken and displayed nearly with the layout you will get if the post is submitted. This preview is not updated. If you like to have a new snapshot, uncheck and check again the box of this preview.
I was not able to test these previews on all browsers and operating systems. Therefore I ask you for your experience here. You may also give ideas of change requests or additional requirements for previews.