By default the preview frame's content is editable, but beware the input elements in the frame are NOT working.
If you do not see updates in the editor after making modifications inside the preview frame try reloading the page.
Because of Cross Origin Restrictions you will be able to edit only files saved in the same directory (or sub-directories) of this file, or those saved in the local Storage.
If, after restoring a file from local Storage, the page does not appear in the preview frame try reloading the page.
If OK button is disabled in downloading requester, try activating the request window by clicking inside it.