During development of web Frontend you can see a huge amount of CSS frameworks which you can use for simplification of your development.
But which you should really use? That can be very hard to answer this question.
I decided to try answer it by comparison of all frameworks what exists right now.

Here is my list:

Bootstrap

Latest stable version: 5.1.0
Github stars: 152K
Development: active
Long of active development: 10 years
Active maintainers: 2
Contributors: 1236
Number of commits: 21K
License: MIT
Used by: 2,662,212 Github projects
Reddit followers: 12.4K

Foundation

Latest stable version: 6.6.3
Github stars: 29.1K
Development: without activity for last 1 year
Long of active development: 8 years
Active maintainers: 0
Contributors: 1028
Number of commits: 17K
License: MIT
Used by: 24 Github projects
Reddit followers: -

Bulma

Latest stable version: 0.9.3
Github stars: 44K
Development: low activity
Long of active development: 5.5 years
Active maintainers: 1
Contributors: 362
Number of commits: 1631
License: MIT
Used by: 160K Github projects
Reddit followers: 71

Tailwind CSS

Latest stable version: 2.2.7
Github stars: 45.4K
Development: quite good activity
Long of active development: 3 years
Active maintainers: 1
Contributors: 184
Number of commits: 4K
License: MIT
Used by: 265K Github projects
Reddit followers: 4.5K

HTML5 Boilerplate

Latest stable version: 8.0.0
Github stars: 51K
Development: low activity
Long of active development: 11 years
Active maintainers: 2
Contributors: 258
Number of commits: 2123
License: MIT
Used by: 2.1K Github projects
Reddit followers: -

Metro UI

Latest stable version: 4.5.0
Github stars: 6.8K
Development: very active
Long of active development: 6.5 years
Active maintainers: 1
Contributors: 51
Number of commits: 3010
License: MIT
Used by: 173 Github projects
Reddit followers: -

UIkit

Latest stable version: 3.7.2
Github stars: 17K
Development: active
Long of active development: 6.5 years
Active maintainers: 2
Contributors: 32
Number of commits: 4606
License: MIT
Used by: 8.8K Github projects
Reddit followers: 17

Materialize

Latest stable version: 1.0.0
Github stars: 38.6K
Development: very low activity
Long of active development: 4 years
Active maintainers: 0
Contributors: 261
Number of commits: 4K
License: MIT
Used by: 76.2K Github projects
Reddit followers: 22

MUI

Latest stable version: 0.10.2
Github stars: 4.4K
Development: without activity
Long of active development: 4 years
Active maintainers: 0
Contributors: 28
Number of commits: 647
License: MIT
Used by: 2.4K Github projects
Reddit followers: -

Cirrus

Latest stable version: 0.6.2.1
Github stars: 812
Development: active
Long of active development: 4,5 years
Active maintainers: 1
Contributors: 8
Number of commits: 904
License: MIT
Used by: 0 Github projects
Reddit followers: -

Montage

Latest stable version: 17.2.1
Github stars: 1.5K
Development: without activity for 1,5 years
Long of active development: 7 years
Active maintainers: 0
Contributors: 35
Number of commits: 5697
License: BSD
Used by: 0 Github projects
Reddit followers: -

Pure.css

Latest stable version: v2.0.6
Github stars: 21.8K
Development: very low activity
Long of active development: 1.5 years
Active maintainers: 0 (project is finding maintainer)
Contributors: 64
Number of commits: 659
License: BSD
Used by: 4.1 Github projects
Reddit followers: -

Semantic UI

Latest stable version: 2.4.1
Github stars: 49.5K
Development: without activity for 3 years
Long of active development: 6 years
Active maintainers: 0
Contributors: 201
Number of commits: 6684
License: MIT
Used by: 12.6 Github projects
Reddit followers: -

Fomantic UI

Latest stable version: 2.8.8
Github stars: 2.6K
Development: active
Long of active development: 9 years
Active maintainers: 1
Contributors: 252
Number of commits: 8K
License: MIT
Used by: 1K Github projects
Reddit followers: -

Material Components Web

Latest stable version: v12.0.0
Github stars: 15.7K
Development: active
Long of active development: 8 years
Active maintainers: 2
Contributors: 380
Number of commits: 6835
License: MIT
Used by: 6K Github projects
Git repository: https://github.com/material-components/material-components-web

Material Design Lite

Latest stable version: 1.3.0
Github stars: 31.8K
Development: without activity for 5 years
Long of active development: 2 years
Active maintainers: 0
Contributors: 178
Number of commits: 2872
License: Apache
Used by: 11.1K Github projects
Git repository: https://github.com/google/material-design-lite

Skeleton

Latest stable version: 2.0.4
Github stars: 18.3K
Development: without activity for 7 years
Long of active development: 1 years
Active maintainers: 0
Contributors: 25
Number of commits: 167
License: MIT
Used by: 0 Github projects
Git repository: https://github.com/dhg/Skeleton

Conclusion

So from 17 the most known CSS frontend frameworks we can choose only 9 projects which are under active mainteinance yet:

Probably on the first place is Bootstrap which have the most active developers and community around this project.
On second and third place are UIkit and Material Components Web which are maintained very well with two maintainers.

But if you want to use some alternative to this 3 top frameworks, you can choose:

• Metro UI -> if you want to have design like Metro UI in Microsoft Windows
• Tailwind CSS -> where you can style your project in very different way
• Cirrus -> if you want to have some lightweight framework
• Fomantic UI -> if you want have successor for Semantic UI

Also you can try Bulma or HTML5 Boilerplate but only for personal home made projects because there is not so big development like in prevous frameworks and is here a risk that it become dead after some years like other frameworks from my list.