Media types are mutually exclusive in the sense that a user agent can only support one media type when rendering a document. However, user agents may use different media types on different canvases. For example, a document may (simultaneously) be shown in ‘screen’ mode on one canvas and ‘print’ mode on another canvas.
@media and @import rules with unknown media types (that are nonetheless valid identifiers) are treated as if the unknown media types are not present. If an @media/@import rule contains a malformed media type (not an identifier) then the statement is invalid.
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