I think HTML is exactly the "client-side multimedia file format" you want. I guess what we don't have is an established editor UI. You have to create it yoursel

I think HTML is exactly the "client-side multimedia file format" you want. I guess what we don't have is an established editor UI. You have to create it yourself. 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.