Kinds of CSS Statements
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At-rules They start with an at sign (@), followed by an identifier to say what kind of rule it is, then a syntax block of some kind, ending with a semi-colon (;). Each type of at-rule, defined by the identifier, will have its own internal syntax and semantics. Examples include: @charset and @import (metadata) @media or @document (conditional information, also called nested statements, see below.) @font-face (descriptive information)
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Nested statements They are a specific subset of at-rule, the syntax of which is a nested block of CSS rules that will only be applied to the document if a specific condition is matched: The @media at-rule content is applied only if the device which runs the browser matches the expressed condition; the @supports at-rule content is applied only if the browser actually supports the tested feature; the @document at-rule content is applied only if the current page matches some conditions. Specific syntax example: @media (min-width: 801px) { body { margin: 0 auto; width: 800px; } }