Sassy CSS, or SASS, expanded the feature set of CSS, though a lot of what it offers has now been integrated into the CSS spec, like CSS Variables, Nesting and scoped styles through CSS Modules.

However, legacy codebases can often still use SASS, or competitor LESS, and some features and implementations remain more robust in SASS, like deep nesting, partials, mixins, inheritance, loops and functions.

SASS still has plenty to offer, and I consider it a useful tool for managing CSS in my projects.