That frontend code already exists and does that every time you vote (line 27 of https://news.ycombinator.com/hn.js). The response content isn't used for anything at all.
Nice! I hadn't actually looked at the code before writing my comment. That does seem relatively trivial, so I stand corrected. I didn't realize the clientside code was discarding the response; I sorta naively assumed it was doing a DOM replacement or something equally straightforward.
What a pleasantly tiny script file. More site code should be this easy to read.
If you have JavaScript disabled, the response content is used to hide the upvote sign.