Not sure it's theoretically possible. What you can do in an interactive WYSISYG editor like a word processor and what you can do with batch typesetting program

Not sure it's theoretically possible. What you can do in an interactive WYSISYG editor like a word processor and what you can do with batch typesetting program are two different things. When you edit in a word processor, you have different expectations about the interaction speed, and the locality of changes. A typesetting program can spend two seconds optimizing word spacing when you "render" the document. Word can't. It could spend those 2 seconds when you hit print - but then it's no longer WYSIWYG! It could also apply "TeX-like" typesetting changes (jumbling the spacing around in the whole document on every keystroke) but that also breaks the user's expectation.