We might have pushed CSS further than it should go, but using gradient for the connector lines is pretty clever.
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 remember various triangles using "border" and 0 width/height being pretty common even in the decade before that.
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.