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An app.
Support for js is in the epub3 spec as well as other stuff best only done in apps or in a browser.
Actual ereaders apps mostly only do a subset of epub3, if at all. Most eink readers sold (by volume) don't do epub3 at all and the few that do, only do a subset. Most eink does any animation poorly and few have colour (max about 4,000 variations compared to 32,000 to 16,000,000+ on other screens).
Slightly more than 1/2 of ebooks are read on apps rather than eink, but screen size varies from about 4" to 13" for phones and tablets.
Many feature in epub3 do little for books and some features needed by books are not in it. It was created when epub management was folded into the group that manages HTML & CSS for browsers, so lost focus on books.
Video, interaction, animation and even colour etc is best done using a free off the shelf frameworks for iOS and Android Apps.
Anything needing a fixed layout or a minimum screen size is often needing a 8″ or larger tablet.
There are three or four epub3 enthusiasts here content to produce content that won't work for about 95% of people that buy ebooks with English (Amazon has over 92% of the English ebook market and epub is only an upload format for them that gets converted to mobi, azw, azw3, and KFX for the actual Kindle or app that a customer has).
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