I wonder what modern(ish) CSS features can be relied on to actually work in ebook readers. For example, can you use flexbox to build page layouts that are responsive to screen and font size?

Kobo has two renderers, one based on QtWebKit 5.2, the other on a (fairly old version of) Adobe RMSDK, both from around 2013-2014. And RMSDK doesn't correctly implement the CSS spec - it throws out the entire stylesheet on a parser error instead of only the declaration which failed to parse.

From that cutoff date, I can *maybe* use flexbox in the Kobo Qt renderer, but if so it would only work with the "-webkit-" prefixed properties, and those would break the styling in RMSDK.