Sorry, I'm late to the game on this one! I'd argue for including additional rules that make practical sense to this section and allowing for a variety of ways to format certain kinds of text and information.
If we find that most pages are or aren't following a guidelines or using completely different ones not documented here, we should revise the guide to fit the most common practices. I would personally fall on the side of the easiest, most commonly accepted and privilege those as preferred in the guide -- which I would argues is more a common set of best practices rather than a perfect standard.
I know that this might seem contrary to the idea of a guide or guidelines or rules, but we are working together on wikis with many contributors from many different backgrounds of varying abilities and skillsets. The goal here is not perfection but clear understanding :-) We can do our best by outlining best practices (but we may find there is a better or more common practice).
@Quiddity and others Thank you for making changes and additions to the formatting standards :-)