Two notable and widely used examples are Bootstrap and Foundation.
CSS frameworks offer different modules and tools:
- reset style sheet
- grid especially for responsive web design
- web typography
- set of icons in sprites or icon fonts
- styling for tooltips, buttons, elements of forms
- parts of graphical user interfaces like accordion, tabs, slideshow or modal windows (Lightbox)
- equalizer to create equal height content
- often used CSS helper classes (left, hide)
Bigger frameworks use a CSS interpreter like Less or Sass.