Table and Frames - how layouting dominated the CSS school of thought
Grids and Frameworks - the mighty grid frameworks that gave you lots of stuff for free at the cost of the language itself (and your freedom)
CSS Themes - how SASS, naming conventions, and framework customization created the era of CSS theming
CSS Components - how we realized everything we did in the past was wrong, fixed up the language itself, and made styles servants to modular components, the way it should have been all along (spoiler alert: Javascript helped).