Didn't normalize.css just get rid of opinionated defaults? I don't see how this would be less opinionated then the old font-family: sans-serif; rule.

This is opinionated. See this thread #664

Using system fonts is not "opinionated".
System fonts are just system fonts.
If you insist, use:
font-family: -apple-system,system-ui,BlinkMacSystemFont,"Segoe UI",Roboto [Removing the "Helvetica Neue",Arial,sans-serif part]
This is normalization, not reset.
Again- this solution is used by GitHub, Wordpress, Bootstrap, Medium, Ghost, etc.

What browser is using ANY of these fonts by default?

Read:
Font stack in 2017

@DoronBrayer that does not answer @miquelfire's question:

What browser is using ANY of these fonts by default?

Normalize relates to default styling and—as far as I know—browsers do not use system fonts out-of-the-box.

If 99% of the websites uses sans-serif as default, why go with the other 1%?! - Go with the majority, go with the 99%!
In addition - sans-serif is used by any framework out there, and there's a reason for that.
Not setting a font-family (in body or html) is so 2007.
It's time to move on, really.

@DoronBrayer, I get what you are saying, but what you are describing is less the use case for bugs and common browser inconsistencies and more the use case for common developer expectations and preferences, like box-sizing: border-box, etc..

At least one project needs to hold the fort so to speak so that there may be a reliable base for your own projects, or bootstrap, or sanitize.css, etc.

Noting for further reference, that we've had to back out system-ui over at Bootstrap due to a number of issues with faulty Chinese fonts on Windows.

Since this feature isn't going to be added here, a shameless plug: https://github.com/mvasilkov/systematize
It's basically normalize.css with some improvements to typography, coming from this repo's pull requests mostly. WIP.

put "Segoe UI Emoji" before "Segoe UI Symbol". otherwise, chrome/edge in windows will only display the emoji in white/black color.

FWIW here’s the current CSS I’m using to normalize emojis:

// Custom emoji font-family to standardize appearance across platforms
// https://www.client9.com/css-color-emoji-stack/
// https://nolanlawson.com/2022/04/08/the-struggle-of-using-native-emoji-on-the-web/
@font-face {
  font-family: "color-emoji";
  src: local("Apple Color Emoji"),
       local("Twemoji Mozilla"),
       local("Segoe UI Emoji"),
       local("Segoe UI Symbol"),
       local("Noto Color Emoji"),
       local("EmojiOne Color"),
       local("Android Emoji");
}

Then you can append color-emoji to the end of your font stack:

body {
  font-family: system-ui, Segoe UI, sans-serif, color-emoji;
}

But My question is How a font become heaviest , largest, painful resource for a website ? ?

But My question is How a font become heaviest , largest, painful resource for a website ? ?

There are many glyphs in a font for each character, those glyphs are basically vector graphics united together. A typeface has multiple styles of glyphs... Yeah, it depends on how complex the font is.