Semantic UI is a MIT open source user interface library that provides the CSS needed for clean content sites and applications. Most of the CSS on this site is Semantic UI with minor additions for site needs.

Example: Here are some sample elements from the UI library.

Markup can be used in HTML directly, in mustache templates as well as using wrapper controls.
Also there are Block controls for WordPress and ActionApp native controls and panels that use these elements extensively.

Find the full library here https://semantic-ui.com/. Want an easy playground? Create a HTML resource item in the system and that provides a HTML editor and preview pane, no setup needed to get started.