While I have toned down from my Ratpoison/fbi/libcaca days, I still use Vim. This led me to Refined Hacker News [1]. Unfortunately, it looks like it is now unmaintained and does have some annoying limitations.
So, I finally made my own browser extension for Firefox and Chrome. It is primarily focused on keyboard-based navigation, should introduce no visible delay, and minimize surprises.
It has many Vi-like key bindings to navigate comments (not just j and k). I'll let you take a look at the README for the list!
Also, I stole the idea from bakkot's SlateStarComments [2] to quickly find new comments. I never quite liked tracking the user's last visit for this. Instead, you can easily browse the comments in reverse chronological order and stop when you get to those you already read.
Among other features, you can quick-reply/edit/delete comments, preview comments when replying/editing, it highlights op, and it spreads request in time to handle the fact that Hacker News 503s if you go too fast.
The extension is written is vanilla JavaScript [3], with no build step. Although I like strong typing, I found that type annotations added more friction than they found bugs, at least for small projects, so no JSDoc either. This results in a single 1k LoC file that I can mostly keep in my head, and no package.json or vendored library.
[1] https://github.com/plibither8/refined-hacker-news