Epub Code RSC-004

1) Validators are quite variable and, truth, not really of much use unless you are hand-assembling your EPUBs. EPUBs exported from a reliable creator like InDesign either work or they don't; minor validation warnings are irrelevant to 95% of the downstream.

2) Validators being variable, the only one really worth paying attention to is EPUBcheck. A bit tricky to use, since it's a command-line tool, but it's the only one that's up to date and standards-based. It is often used as the "engine" of other validators... but nearly all of those impose their own checks and preferences, making the validation questionable.

3) ADE is absolutely useless. Never use it to view EPUB, much less proof new work. Use a vanilla, standards-based reader like Thorium, with Calibre being a good second. All of the rest are broken, proprietary or otherwise poor choices to check a new export.

4) If it's a reflowable EPUB, don't embed fonts. Furthermore, don't spec fonts for EPUB at all.

5) If it's a fixed-page EPUB (FXL), you don't have the option to avoid embedding fonts, but that's just one of the many reasons to avoid FXL altogether. If you want EPUB, go with reflowable and an export that any reader can manage without interference like embedded fonts. If you want fixed page export, use PDF instead.

FXL ePub or Reflowable ePub?

Try changing the font to an OTF one.

What do you mean by "a mess"?