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”.
Ah, well, no. Lots of people use single letter prefixes for CSS. The global company that I work for does exactly this. Famous web authors use it.
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.
You can read more about this terrible aspect of microformats.
I like microformats. I want to use them more. But they feel horribly in collision with CSS authoring practises.