font-smoothing: antialiased;
-webkit-font-smoothing: antialiased;
-moz-osx-font-smoothing: grayscale
① I believe this only affects macOS, and has no effect on any other platform.
② There’s no such property as font-smoothing. Only -webkit-font-smoothing and -moz-osx-font-smoothing, with their different values.
③ Attempting to control this is highly controversial. You may prefer it one way, but others may prefer it otherwise. See, for example, https://usabilitypost.com/2012/11/05/stop-fixing-font-smooth.... (Note that discussion of these things tends to be from 7–15 years ago. I have no idea what the present situation is; I know macOS largely eschews subpixel antialiasing these days, so maybe the effect of this property has changed now, or maybe it literally does nothing, I don’t use macOS.) As a generalisation, you should not do things like this, but leave it to the platform or the user to decide. Features like this don’t tend to end up part of the standardised web platform, because you normally shouldn’t be able to control them, and when people control them, they’re normally doing so in order to work around platform-specific issues, or just because they’re copying and pasting what someone else did without understanding.
> As a generalisation, you should not do things like this, but leave it to the platform or the user to decide.
Tailwind also does it (e.g. see the CSS behind this page [2]). Those guys wrote probably my all-time favourite resource on UI design [3], so I do put value on little tricks like this that they're using.
1. https://developer.mozilla.org/en-US/docs/Web/CSS/font-smooth