Reveal Codes is a good start, but if I remember correctly they were read only. you could not close the loop and enter codes.

Reveal codes in a small way made me swear off word processing for good. We had an office document in wp 8 or 9 if remember correctly, you know the type, it lived on a shared drive and whoever needed to make changes did. anyhow it was a complete mess, internal corruption all over the place, no consistent style. one night I sat down with it and thanks to reveal codes I was able to clean it up. At which point I had a revelation, most of the sins of the document were due to the WYSIWYG interface. I was able to fix it because I could(almost) see the source. Why not cut the middleman write the document in source and render it to the final form. So I rewrote it in html. and have had a lingering resentment of word processors ever since.

You know why they call them word processors right?... Ever see what a food processor does to food?