I don't think we pushed CSS too far; the complexity of CSS is inherently tied to the complexity of the UI requirements. Even now CSS can't handle everything.
Well, they had a dominating browser for many years, so they know the benefit. Apparently they decided it is cheaper to just focus on the "user experience".
Now they can still control what buttons there are and whatbis shown on the home screen by default, but do not have to bother so much with all the expensive technical details.
I suppose the lines won't be interactive (e.g. "double click on this line to add a text label") because the line isn't there in any sense Javascript would be aware of. Unless Javascript computes the same gradient, which is then a pointless duplication. But it's very clever.
Makes me wonder if there's a missing format out there that should be built into browsers that describes shapes and connections.