Having partially reviewed a couple of these, I'd say quality varies quite a bit so be sure to dig through the code to see if you'd be comfortable helping maintain that. Always be sure to review the license! For example, Microsoft Fabric is MIT, but the assets (fonts and icons) are under a separate license.
No real intent there. It's good to have large organizations donating libraries to open-source.
Google's https://material.io/design/
I guess most companies have a design system in place
If you want style guides and design systems then the link by "oedmarap" is much better.
https://www.quora.com/What-CSS-framework-does-Twitter-use-in...
Other sites tend to have rendering issues with some lines in letters either becoming doubly thick or barely visible, white text pixelating out to bright backgrounds, anti-aliasing turning some pixels colorful or too dim etc. Some of them seem to be exposing edge cases of text rendering! Text could be too gray to read or too thin.
Designers need to consider people still buying laptops with 1366X768 screens.
https://news.ycombinator.com/item?id=18235887
https://element.eleme.io/
https://ant.design/
https://quasar-framework.org/
https://at-ui.github.io/at-ui/
https://developer.microsoft.com/en-us/fabric
https://vmware.github.io/clarity/
http://appnexus.github.io/lucid/
https://ng-lightning.github.io/ng-lightning/
https://blueprintjs.com/
http://www.jetbrains.org/ring-ui/
https://www.lightningdesignsystem.com/
https://elastic.github.io/eui/
https://atlaskit.atlassian.com/