One of the most complicated sections is the table in Documentation/Style guide#Formatting Text. There is widespread variation in external style guides, and inconsistent usages throughout our own documentation, especially in user-oriented docs vs developer-oriented docs.
I've attempted an overhaul, using this 2018 diff and the older 2015 diff that we started with, plus consulting the HTML spec and some of the external style guide examples.
I'm hesitant about some of the changes, and hope to discuss the pros/cons some more with you all. If you know of any exemplary in-current-use examples to aid in that discussion, that would be great. Aiming to be "descriptive not prescriptive", as much as possible.