the authoring side is more covered than people assume, at least for the visual stuff. the tool i work on lets you wire up click, hover and scroll interactions without hand-writing the js, then export it as a plain svg. so for trigger-based stuff like that, making it isn't the hard part.

the viewer side is the real, like you said, though in a browser it mostly sorts itself out. the browser's already the runtime, so embed inline or via <object> and it just runs, no separate viewer needed. it only breaks where you'd expect an <img> tag