> But sometimes—and here's where it gets uncomfortable—you actually do need a button that updates part of a page without reloading the whole damn thing. You do need a search box that shows results as you type. You do need interactivity.

You can do this with plain old Javascript. Make a request, swap out the [inner | outer]HTML with the result. If you want a nice visual transition, wrap the swap in a startViewTransition(). Obviously, you need to be extra careful if you're using user-submitted HTML. Otherwise, it's fairly straight forward.