Much of Sass’s extra functionality comes in the form of new at-rules it adds on top of CSS:
@useloads mixins, functions, and variables from other Sass stylesheets, and combines CSS from multiple stylesheets together.
@forwardloads a Sass stylesheet and makes its mixins, functions, and variables available when your stylesheet is loaded with the
@importextends the CSS at-rule to load styles, mixins, functions, and variables from other stylesheets.
@includemakes it easy to re-use chunks of styles.
@extendallows selectors to inherit styles from one another.
@at-rootputs styles within it at the root of the CSS document.
@errorcauses compilation to fail with an error message.
@warnprints a warning without stopping compilation entirely.
@debugprints a message for debugging purposes.
Sass also has some special behavior for plain CSS at-rules: they can contain interpolation, and they can be nested in style rules. Some of them, like
@supports, also allow SassScript to be used directly in the rule itself without interpolation.