> The only way it can truly not affect anyone else is if no one else knows that you use ligatures
That covers 99.999999999% of the times ligatures are used then, that's my point.
Sure, in a situation like screensharing, using ligatures "affects someone else". A font also "affects someone else". A theme also "affects someone else". Night/dark mode also "affects someone else". That's a given. It's also something that is normally trivial to turn on or off.
And sure, very, very occasionally people use custom fonts with ligatures turned on in the CSS on websites.
> I think it’s fairly clear that for most people in most circumstances, a light background is superior to dark, and ligatures are a bad idea.
It's not clear at all. Regarding typefaces and character sets and reading comprehension, what a person is used to reading turns out to be the easiest thing for them to read (why do you use a monospaced font in your text editor?). I get that it's a fun thing to argue about because it is completely stylistic, context-sensitive and personal and has near-zero real world effects (I'm doing it now!)
> But ligatures change the actual glyphs, so that your fingers must type something other than what your eyes see, and any viewer must decode the ligature’s meaning.
As opposed to typing a collection of "actual glyphs" which the viewer must then decode back to their actual meaning via the conventions of the language? The characters available a given programming language are determined by [sensible] technical limitations, so creative use needs to be made of those to represent concepts that are already represented by single, well known characters that are not available in the PLs character set.
My beef is that talking about which fonts you like/dislike: that's a productive discussion as they're a personal thing, very often an aesthetic choice.
But banket saying "usage of this _type_ of font is bad": that's not productive in any way, it's pointless, as it ignores the huge % of people who clearly don't share this view. Saying "it's fairly clear ligatures are a bad idea": no, it's display preference, and if is forced on you, you will generally have agency to request it be changed back to something you are used to.
And this is every time coding fonts appear in HN: comment after comment stating the latter as if it's some empirical truth when it most definitely is not (and yes, similar thing for tabs Vs spaces etc, so obviously everyone paints the bikeshed, but this time it's completely tied to one person's personal computer display)