The “advanced” version is what we produce by default, before it’s processed for consumption by older readers. So, to give a couple of examples, CSS pseudoclasse

The “advanced” version is what we produce by default, before it’s processed for consumption by older readers. So, to give a couple of examples, CSS pseudoclasses are processed into actual classes, and MathML is rendered as PNGs. The hypothetical perfect ereader could support this natively, but for most ereaders its going to be a less buggy experience to go for the version with compatibility shims. (Incidentally, Kobo has an excellent MathML engine, so we don’t do the aforementioned PNG rendering for kepubs.)