W3C CSS2 Cascading Style Sheets, level 2 - instructional media + ...

When a style sheet resides in a separate file, user agents must observe the
following priorities when determining a document’s character encoding (from
highest priority to lowest):
1. An HTTP "charset" parameter in a "Content-Type" field.
2. The @charset at-rule.
3. Mechanisms of the language of the referencing document (e.g., in HTML,
the "charset" attribute of the LINK element).
At most one @charset rule may appear in an external style sheet -- it must not
appear in an embedded style sheet -- and it must appear at the very start of the
document, not preceded by any characters. After "@charset", authors specify the
name of a character encoding. The name must be a charset name as described
in the IANA registry (See [IANA]. Also, see [CHARSETS] for a complete list of
charsets). For example:
Example(s):
@charset "ISO-8859-1";
This specification does not mandate which character encodings a user agent
must support.
Note that reliance on the @charset construct theoretically poses a problem
since there is no a priori information on how it is encoded. In practice, however,
the encodings in wide use on the Internet are either based on ASCII, UTF-16,
UCS-4, or (rarely) on EBCDIC. This means that in general, the initial byte values
of a document enable a user agent to detect the encoding family reliably, which
provides enough information to decode the @charset rule, which in turn determines
the exact character encoding.
4.4.1 Referring to characters not represented in a character
encoding
A style sheet may have to refer to characters that cannot be represented in the
current character encoding. These characters must be written as escaped [p. 38]
references to ISO 10646 characters. These escapes serve the same purpose as
numeric character references in HTML or XML documents (see [HTML40], chapters
5 and 25).
The character escape mechanism should be used when only a few characters
must be represented this way. If most of a document requires escaping, authors
should encode it with a more appropriate encoding (e.g., if the document
contains a lot of Greek characters, authors might use "ISO-8859-7" or "UTF-8").
Intermediate processors using a different character encoding may translate
these escaped sequences into byte sequences of that encoding. Intermediate
processors must not, on the other hand, alter escape sequences that cancel the
special meaning of an ASCII character.
Conforming user agents [p. 32] must correctly map to Unicode all characters in
any character encodings that they recognize (or they must behave as if they did).
For example, a document transmitted as ISO-8859-1 (Latin-1) cannot contain
Greek letters directly: "κουρος" (Greek: "kouros") has to be written as
"\3BA\3BF\3C5\3C1\3BF\3C2".
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