greasyphalanges

By doing it in CSS you eliminate the need to re-render your DOM and make your JavaScript do more useful tasks, and make your website more resilient to scripts crashing, freezing, or being disabled by the user. Also, one would think that a pipeline of JavaScript + rendering engine falls short to a more direct solution, as browsers cheat by not using JavaScript ;)

I will concede to an argument that CSS exemplifies "design by committee" pejorative and is blighted by backward compatibility woes, but rest assured, it does its job as best as it can. Web development has patchworked its way out of most problems it has created, making it somewhat burdensome to learn. I'd say that is its Achilles's heel.

Do all solutions require you to have end nodes be connected to a single graph or there exists a puzzle where the solution has disjoint graphs? I'd also specify that no hanging pipes are allowed / utilize all pipes, although that part becomes obvious pretty quickly.