• Fluent (Microsoft) - https://www.microsoft.com/design/fluent/#/
• Material (Google) - https://material.io/design
• Bootstrap (Twitter) - https://getbootstrap.com/
• Carbon (IBM) - https://www.carbondesignsystem.com/
• Foundation - https://get.foundation/

Lots more too. Add as needed

• CSS New Features - https://blog.gitbutler.com/the-great-css-expansion