The syntax of <custom-ident> is similar to CSS identifiers (such as property names), except that it is case-sensitive. It consists of one or more characters, where characters can be any of the following:

• any alphabetical character (A to Z, or a to z),
• any decimal digit (0 to 9),
• a hyphen (-),
• an underscore (_),
• an escaped character (preceded by a backslash, \),
• a Unicode character (in the format of a backslash, \, followed by one to six hexadecimal digits, representing its Unicode code point)

Note that id1, Id1, iD1 and ID1 are all different identifiers as they are case-sensitive. On the other hand, as there are several ways to escape a character, toto\? and toto\3F are the same identifiers.

nono79        A mix of alphanumeric characters and numbers
ground-level  A mix of alphanumeric characters and a dash
-test         A dash followed by alphanumeric characters
--toto        A custom-property like identifier
_internal     An underscore followed by alphanumeric characters
\22 toto      A Unicode character followed by a sequence of alphanumeric characters
bili\.bob     A correctly escaped period

34rem     It must not start with a decimal digit.
-12rad    It must not start with a dash followed by a decimal digit.
bili.bob  Only alphanumeric characters, _, and - needn't be escaped.
'bilibob' This would be a <string>.
"bilibob" This would be a <string>.

• <custom-ident>
• <dashed-ident>