reserve all other single-letter-dash prefixes for future use. In practice we have seen very little (if any) use of single-letter-dash prefixing of class names by web developers/designers, and thus in practice we think this will have little if any impact/collisions. Certainly far fewer than existing generic microformat property class names like "title", "note", "summary".
I love the Indieweb, but the "microformats as HTML classes" thing is just something I find really hard to get on board with. They could have used
data attributes. They could have used a blob of JSON in the head of each page, like schema.org does. But no, they went with something that directly collides with presentational CSS.
I like microformats. I want to use them more. But they feel horribly in collision with CSS authoring practises.