Somewhat familiar with them, but can't say I've ever built anything with them. I'm curious though: are you aware that people already consider React and its cousins to be immediate mode style APIs for the Web? At least, as close as you can reasonably get when your rendering target is the DOM? That's why `UI = f(state)` is so often cited in React literature; components are functions that don't know anything about the "widget tree" or "instances" or anything like that, you always just return the latest output that you want with the data/input you receive, as if you are rendering "from scratch" each time. Components don't really know whether they're "initializing" or "updating" or anything like that, they just know they're "rendering"; React takes care of reconciling that output with the actual DOM.

So, if that's not good enough for you, what would an even more faithful immediate mode API even look like? (And if your idea includes ditching the DOM as the render target, which is inherently "retained mode" and could never be otherwise due to performance and DOM state (like focus), I'm afraid it's a nonstarter.)