If you're looking at this sort of thing from a research perspective, you might be interested in:
I was looking for more @elibarzilay’s writing about Scribble, as the things I was remembering weren’t discussed in detail in “The Scribble Reader: An Alternative to S-expressions for Textual Content”. The most relevant thing I remembered is an old mailing list thread:
Separately, for a more thorough introduction to these parts of Racket, you might consider: