The capability is nice to have—for example, if your website is a coding tutorial website, and you have interspersed code examples and prose, put the code examples in a fixed width font. But it is over-used. For example, why do sites pick serif vs non-serif? Leave it up to my browser.
This isn’t just superficial, some people might use certain fonts that are easier to read for dyslexia, and I don’t think I should make their life artificially harder if it’s trivial for me to simply send a message as plain text.
As for accessibility, the people who need that have already set up the required font overrides and other stuff so it doesn't really benefit them much to use plaintext.
It's sad to think about how things could have been, but that's not the world we live in now...
Also I like using text because it shows I’m not hiding any kind of tracking images or anything like that.
Personally, I quite like the site's design and its font. My gripe often is light gray text on a darker gray background. The bad readability that so many newer sites seem to prefer makes me question my eyes or my monitor capabilities. Reader mode in Firefox is also often very helpful.
But, the feature is overused, IMO. Anything can be used to express a bit of personality, but I do think it is sometimes specified in cases where it really isn’t.
IMO a nice serif font is ideal for long form content though. I remember reading the serifs help guide ones eyes into the next character and create more unique shapes than sans or monospace.
It's the same for mono vs proportional spacing. You are better at reading that which you have the most practice with. Most people are not used to reading monospaced prose even if they have seen a lot of monospaced code.
I've noticed that too - I read code all day, but there's something very odd about having conversations (prose) with Claude Code via a terminal window.
That said, I'd probably just stick to "sans-serif" and let the browser/os preference hold. It's likely a helvetica/arial alike anyway and can be set by user preference if really wanted.