For example, I once adjusted an Object Pascal interactive program (target: Windows/Win32) for the browser target (FreePascal compiler has the JS target). An intermediate result was a bunch of files that worked locally on desktop but struggled on mobile. With a little help from the SingleFile extension [2], I ended up with a single HTML file containing all functionality and content. It worked great, for example, in MiXplorer's internal HTML viewer. I can't recall the exact details, but the file:/// protocol still had issues in Chrome, Firefox, or both. Anyway, preparing a local address correctly with a keyboard is a challenge so let's just assume that having capable file managers running local html files is enough
Sure, to make this manageable, you need good tools that handle all sides of the task. But at least in theory, the format is fully capable. My only global issue was that the state for locally run HTML files is a kind of ephemeral entity, but for interactive multimedia files, you may consider this obstacle small.
Most of all it needs usable editors, and editors which integrate multimedia and dynamic content editing. End users can't turn to a different editor for each media and then integrate the output into the epub document, like a web developer does (e.g., for an image use Photoshop, save the jpg, copy to the proper directory, reference appropriately in the html).
It's if we had the .docx format but MS Word was read-only. You would have to create the XML and zip it yourself, to be then rendered by Word. That's effectively how I see HTML+js in browsers.
