[KevinH]

You may be better off making a web digital publishing project that will be hosted on your own server, than trying to shoehorn things into an epub, unless you only want to target one specific platform like Apple's Books which have looser specifications in many instances. Or a full web application. Things to think about.

I see from your first post, you already do that. Is there some reason you want to constrain yourself with what works in epub3?

Sigil has a full blink webkit engine inside it. So Sigil accepts much more than what the epub3 spec allows. Detecting when that happens is what epubcheck is all about.