I am sharing an Apps Script file that is a Web App with an HTML Rich Text Editor.
It is based on code provided by Mozilla, with just a few modifications. And of course it is
adapted to be configured to run as an Apps Script Web App.
If you look at it, and see something that can be improved, please make a suggestion, or provide code.
To get a copy of the file, click the FILE menu, and then click the menu item "Make a copy"
The following link will open an Apps Script file in the code editor. The tabs will show a padlock, but you can copy the file
and the copy will be owned by you in your Google Drive.