1) bootstrap 

DEVELOPED BY TWITTER

MOBILE FIRST APPROACH

RAPID PROTOTYPE

LARGE COMMUNITY

2) FOUNDATION

Full tooling

Extreme flexibility

More than just UI components

Training and consulting

3) BULMA

Quite popular

Extremely readable classes

Flat learning curve

Elegant

4) UIKIT

Super-Clean

Elegant

less size

Minimilistic

5) Semantic-ui

6) susy

7) Materialize

8) pure

9) skeleton

10) milligram