Much like the values of properties, each at-rule has a different syntax. Nevertheless, several of them can be grouped into a special category named conditional group rules. These statements share a common syntax and each of them can include nested statements—either rulesets or nested at-rules. Furthermore, they all convey a common semantic meaning—they all link some type of condition, which at any time evaluates to either true or false. If the condition evaluates to true, then all of the statements within the group will be applied.
Conditional group rules are defined in CSS Conditionals Level 3 and are:
Since each conditional group may also contain nested statements, there may be an unspecified amount of nesting.