What kind of features does "advanced epub" provide that "compatible epub" cannot?

Our projects are built with the broad assumption that the latest tech is supported across the board: the “advanced epub” is generated by zipping that up and renaming it to .epub. The build step then runs the book through a compatibility pass to remove some of the functionality that simply isn’t there in many readers. For example, it:

1. renders the cover as a JPG from the native SVG

2. converts more complicated selectors (e.g. :nth-of-type, :empty, etc.) to classes and class selectors

3. renders MathML as images

4. renders SVGs as images

…and a bunch more things to make it work better with older ereaders, which tend hang around for a long time.

It’s worth pointing out that the Kobo builds are closer to the “advanced” build as they have great native support for MathML and SVG. But the build step adds in a bunch of Kobo-specific markup to make it work better there.

In my experience, Apple Books does the best with the advanced builds, but it doesn’t support SVG covers so you end up with autogenerated ones, which seems a shame. I’ve sent a couple of feedback issues to them over the years, but if anyone else wants to also do this feel free.