You want PWP <https://blog.jonudell.net/2016/10/15/from-pdf-to-pwp-a-vision-for-compound-web-documents/ https://blog.jonudell.net/2016/10/15/from-pdf-to-pwp-a-v

You want PWP <https://blog.jonudell.net/2016/10/15/from-pdf-to-pwp-a-vision-for-compound-web-documents/ https://blog.jonudell.net/2016/10/15/from-pdf-to-pwp-a-visio...> (Later aborted, and the group's work was rolled into EPUB3. As you note, there remains a genuine need for it.) On the other hand, there's nothing stopping you from using a double-barrelled file extension for denoting this sort of thing, e.g. "memex-opus.pub.html"; so long as it ends with something recognizable, double-clicking should still open it in the browser across all the usual platforms, AFAIK. (I'm fond of using "xyzzy.app.htm" myself to take advantage of this trick for distributing simple, self-contained programs that are designed run in the browser.)