There are some misconceptions in this thread. Hopefully I can clear them up.
First off, SE’s “Advanced epub” is just the default codebase we work on, zipped up. It’s a modern epub3, with no attempt to be bug-compatible with readers beyond meeting the standard spec. When work is finished on the production, it’s zipped up and made available for download.
The “Compatible epub” is the advanced epub, but with some extra processing to make it work better in real life ereaders. The main thing that’s added is a compatibility.css file that has style fixes for common platforms (for example to work around bugs with hanging punctuation in Apple Books). We also render all SVG lineart and MathML formulae to PNGs to get past common missing functionality.
The Kobo kepub has Kobo spans and the .kepub extension applied to trigger the native Kobo renderer. We also apply some of the compatibility fixes, but notably not the MathML prerendering as Kobo’s native MathML renderer is capable.
Finally, the Kindle version is the Kindle version. Not much we can do there.
JSWolf: obviously I don’t want any rendering problems in our epubs. Just out of interest I ran our stylesheets through the CSS Validator to see if I could replicate what you’re seeing, and it did flag some errors. On inspection though, the files themselves don’t contain the problems reported, so it’s a bug with the validator(!)? In future, if you find any more issues and have time, please feel free to PM me, or file them directly at
https://github.com/standardebooks/tools/issues/new . Thanks!