Could this be solved by having two renderers, and only using the proprietary Adobe one needed?

So what do you do when the pedantic gold standard of epubcheck says your book is fine, when it works without issue on Amazon Kindle, Apple Books, Thorium and everywhere else and yet Kobo says it’s “corrupted”?

I dug into this matter and found out that Kobo uses RMSDK, “Reader Mobile Software Development Kit”, Adobe’s proprietary ebook rendering engine.

Once the stylesheet was identified as the source of my woes, I could finally drill down to find which specific line was causing the issue. After creating a dozen more variations with different subsets of my stylesheet I eventually identified the culprit. It was this line:

.copyright img {
    max-width: min(150px, 30vw);
}

Once I changed it to the more old fashioned max-width: 150px; ADE opened it just fine.

But what is the problem here? The above code is perfectly valid CSS level 4, it’s just not supported by RMSDK, because its CSS parser is frozen in approximately 2013 — no flexbox, no grid, no math functions, no custom properties. Just good old float, bad font handling, and silent crashes when it sees anything it doesn’t recognize.

It’s the year 2026. Thanks to the horrendous RMSDK which Kobo decided to use as their backbone for all book rendering (probably for DRM reasons), a single line of perfectly valid CSS turns a perfectly valid EPUB file into a “corrupted file” on Kobo and just drops the whole book. No clear error message, no fallback. Just a massive fail.