About CSS Rules

CSS is made up of properties, functions, at-rules (or @-rules), and more. The CSS specification defines how these features work by explaining the feature, and listing the values or data types that they accept.

An at-rule is a CSS statement beginning with an at sign, @ (U+0040 COMMERCIAL AT), followed by an identifier and includes everything up to the next semi-colon, ; (U+003B SEMICOLON), or the next CSS block, whichever comes first.

For example, the @font-face at-rule allows you to use a custom font so that you aren't solely reliant on the fonts that are installed on the user's computer. And the @import at-rule can be used to import another style sheet into the current style sheet.

While you could easily create a website that doesn't use any at-rules, they do provide extra functionality that could be difficult (or impossible) to achieve otherwise.