In reply to Bradley Kirby.

@Bradley Cheers. I'd say the support is somewhat experimental given that the number of themes implementing could be counted on one hand. There are surely additional things that core can do to facilitate it, so in your investigations please flag anything that is a pain point. One thing I'm aware of is making the selective refresh more straightfoward to adapt for JS-rendered partials. Some of that can be seen in Anadama for nav menus, but I've also been prototyping this for widgets as well in https://github.com/xwp/wp-next-recent-posts-widget/blob/5eee7d3be9b768d5634d8df5def002cd37f2751f/js/widget-view.js#L58-L112