Once a value is passed down as props, you get all that reactivity for free.
More strong connotations of that word are from vocabulary of our southern republican neighbor. Pronunciation is usually accompanied by trumpets and other pompous instruments :)
For html, it's probably much tougher since the DOM is statefull and fares badly with frame-wise changes to it. That might require caching elements to avoid layout invalidation and rebuilding the dom.
A HTML driven imgui sounds cool, I hope Gregg continues building it out.
Of course the irony is in running immediate mode logic through a retained mode UI model. In theory this should be a horrible x horrible idea. Yet I suspect, the results bare out, that web renderers manage.
I'm not convinced this has a benefit over React et al. You could even render react 60 times per second and use only plain values not state if you think that will be beneficial. Virtual DOM and the incredible computing power in our devices will make it work.
I have never worked with real immediate mode gui toolkits though, so I may be missing something nice, e.g. about that sequential declaration style? Or that it feels even less taxing to reason about than react?