When a stylesheet is embedded in a document, its character set encoding is
the same as the document it resides in. If the style sheet is external to
the document, it is assumed to be encoded using Unicode unless otherwise
specified. This At-Rule syntax allows an author to specify the character
set encoding of the style sheet; it should only be used on external style sheets.
The @charset At-Rule is allowed to occur only once in an external style
sheet and it must be the very first statement in the style sheet. This
At-Rule supersedes any encoding declarations specified in the document
that reference the style sheet, but should be over-ridden in the event
that HTTP character set headers ('charset' component of the 'content-type'
syntax) have been specified.