I was lately trying to find an HTML WYSIWYG editor for ABAP, but I failed. I though or this was not needed so far, or the solution was not posted anywhere. So I've tried several times and thanks to NICEdit and this tread on SCN I found the way to make HTML WYSIWYG editor for ABAP.
My editor use CL_GUI_HTML_VIEWER to display NICEdit in container, and then thanks to POST method I put changes back to SAP. ZCL_HTML_EDITOR class, which is attached to this post, raises an event whenever someone click on save button in the editor, so you can easily handle it and then use new HTML for your purposes. Video bellow shows the demo of usage.
Demo program which is included shows how to call the editor and set width and height of it.
Important!: In the zip file you can find  abapblog.com_html_editor.html file which must be uploaded to MIME repository (for ex. via SE80), then you need to update constant mime_path to be able to use editor.