> you just only write JavaScript for the cases where there is a real legitimate benefit to doing so

The snippet above only works because HN is so simple, normally the rule of thumb is if you are going to use JS to change the DOM at all then you need to render it in the first place using JS or else you are double-implementing the render logic and you open yourself up to changing one implementation and not the other. For hiding arrows sure this works but 99.999% of the time it's a very bad idea to use JS to change the dom to a state that the server would otherwise render on a page reload. You may feel that a round trip works better but that's not what users expect or will tolerate most of the time. That kind of thinking feels very close to people who think CRUD is all you need to build an app. Sure you can build an app with just that but thinks break down very quickly.