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At-rules are special instructions for the CSS parser. They are invoked by an at-keyword preceded by an “@” sign.

Specifies the character encoding of the style sheet.
The @font-face CSS at-rule allows authors to specify online fonts to display text on their web pages. By allowing authors to provide their own fonts, @font-face eliminates the need to depend on the limited number of fonts users have installed on their computers.
Imports an external style sheet.
Sets the keyframes for the CSS animation property.
Sets the media types for a set of rules in a styleSheet object.
Declares the default namespace and binds a namespace to a namespace prefix.
Specifies properties of a page box, such as its dimensions, orientation, and margins. This is used for paged media, such as the printed page.
Applies CSS styles to portions of content as it appears when flowing within a specified set of regions.
The CSS @supports at-rule lets authors detect support of CSS features directly in CSS.
Specifies the size, zoom factor, and orientation of the viewport.