letter-spacing is massively overused. I have Firefox set to only use my chosen fonts (and highly recommend it: it makes the web much better), and this reduction in variation makes non-zero letter-spacing much more obvious to me. At typical body sizes, I will tend to notice (and be displeased by) letter-spacing changes of less than 1% (I notice even 0.1px often).

There are only three places where I hold letter-spacing of any kind to be acceptable:

1. Art. (This is pretty much the intended context of the article, but I just know it’s going to be taken out of that context.)

2. In large headings, slightly reducing letter-spacing may be reasonable, as fonts are often spaced for smaller size usage. But whereas reducing line-height is all but essential, reducing letter-spacing is of marginal value at best. Ideally you have a variable font with optical sizing (the opsz axis), so this is handled automatically.

3. In all-caps, small-caps or all-small-caps text, as fonts are generally spaced for runs of lowercase letters. https://practicaltypography.com/letterspacing.html suggests 5–12% (which you can spell in CSS as 5% or 0.05em). Ideally you have a font that optimises this itself within the caps, smcp and/or c2sc features.

Do not, under any circumstances, set non-zero letter-spacing for body text. It’s just wrong. If you feel your preferred font is too spacious or too tight, fix or replace the font.

(If you want another reason: letter-spacing is incompatible with ligatures, not so often used in sans-serif faces, but in serifs things like fi not having the ascender and dot collide messily is often valuable.)