From the wikipedia entry

> WordStar was the first microcomputer word processor to offer mail merge and textual WYSIWYG.

They produced a bunch of different versions, and that screenshot is for an MS-DOS version.

Under Windows 3.1 it had this feature where it would show you WYSIWYG text, but you could ask it to show you the markup as well. It was wonderful, and MS Word can't do it to this present day!