It's not arbitrary though. It's a choice to save bandwidth shipping unnecessary javascript rather than making it easier to develop the website. At the scale of Wikipedia that isn't unreasonable.
There's this pattern on HN: people value a feature as having 0 utility and then become annoyed that someone has paid time/performance/money for them. Well duh, if you discount the value of something to 0, it will always be a bad idea. But you're never going to understand why people are paying for it if you write off their values.
At my last job there were countless pieces of UX to make things smoother, more responsive, better controlled by keyboard or voice reader, etc.. that required JS. It was not possible to make our site as good as possible with CSS, and it certainly wasn't worth the tradeoffs of loading a big faster (not that it couldn't have had it's loading time improved--just, cutting JS was a nonstarter).
The js is unnecessary if you can achieve the same result with plain css.
But to play devil's advocate: just because you can, doesn't mean you should.
In many scenarios I'd argue CSS would require more bandwidth. It can get quite verbose.