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

4.1.2 Keywords
Keywords have the form of identifiers. Keywords must not be placed between
quotes ("..." or ’...’). Thus,
red
is a keyword, but
"red"
is not. (It is a string [p. 50] .) Other illegal examples:
Illegal example(s):
width: "auto";
border: "none";
font-family: "serif";
background: "red";
4.1.3 Characters and case
The following rules always hold:
All CSS style sheets are case-insensitive, except for parts that are not under
the control of CSS. For example, the case-sensitivity of values of the HTML
attributes "id" and "class", of font names, and of URIs lies outside the scope
of this specification. Note in particular that element names are case-insensitive
in HTML, but case-sensitive in XML.
In CSS2, identifiers (including element names, classes, and IDs in selectors
[p. 53] ) can contain only the characters [A-Za-z0-9] and ISO 10646 characters
161 and higher, plus the hyphen (-); they cannot start with a hyphen or a
digit. They can also contain escaped characters and any ISO 10646 character
as a numeric code (see next item). For instance, the identifier "B&W?"
may be written as "B\&W\?" or "B\26 W\3F".
Note that Unicode is code-by-code equivalent to ISO 10646 (see
[UNICODE] and [ISO10646]).
In CSS2, a backslash (\) character indicates three types of character
escapes.
First, inside a string [p. 50] , a backslash followed by a newline is ignored
(i.e., the string is deemed not to contain either the backslash or the newline).
Second, it cancels the meaning of special CSS characters. Any character
(except a hexadecimal digit) can be escaped with a backslash to remove its
special meaning. For example, "\"" is a string consisting of one double
quote. Style sheet preprocessors must not remove these backslashes from a
style sheet since that would change the style sheet’s meaning.
Third, backslash escapes allow authors to refer to characters they can’t
easily put in a document. In this case, the backslash is followed by at most
six hexadecimal digits (0..9A..F), which stand for the ISO 10646
([ISO10646]) character with that number. If a digit or letter follows the
hexadecimal number, the end of the number needs to be made clear. There
are two ways to do that:
1. with a space (or other whitespace character): "\26 B" ("&B")
2. by providing exactly 6 hexadecimal digits: "\000026B" ("&B")
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