Modern CSS framework based on Flexbox – Bulma (by Jeremy Thomas).

From its documentation

CDN

https://cdnjs.com/libraries/bulma

CSS only

Bulma is a CSS framework. As such, the sole output is a single CSS file: bulma.css.

You can either use that file, "out of the box", or download the Sass source files to customize the variables.

There is no JavaScript included. People generally want to use their own JS implementation (and usually already have one). Bulma can be considered "environment agnostic": it's just the style layer on top of the logic.