Hmm, the "With Expression" diagram on line 656 looks fine in emacs, but in chrome at that actual link, everything on the right is mangled. (The characters look fine, it looks like github's font choices do poorly with the "load bearing whitespace" for the extended bits...)
I think it's the ◀ character. It looks like Chrome (I'm on Windows) makes it wider than other Characters. I think ▶ has this same problem. But from a quick scan, I don't see any other place in the file where one of those characters is used in a way that it being extra wide would be noticable.
For web it'll really vary by font family availability and fallback. Personally I like the nerd don't variant for Inconsolata, Cascadia Coffee and Firs Code for fixed width usage.
I wish self packaging similar to how Google Fonts does it was easier to do. IE breaking up the don't into glyph segments to reduce unused characters for languages not used.
I've always wanted to try this but never had a reason: do emojis work on HN? I've never seen one and didn't know if it was just a culture thing or if it was enforced:
