The <custom-ident> CSS data type denotes an arbitrary user-defined string used as an identifier. It is case-sensitive, and certain values are forbidden in various contexts to prevent ambiguity.


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.

Forbidden values

A <custom-ident> must not be placed between single or double quotes as this would be identical to a <string>. Moreover, the first character must not be a decimal digit, nor a hyphen (-) followed by a decimal digit or another hyphen.

To prevent ambiguity, each property that uses <custom-ident> forbids the use of specific values:


Forbids the global CSS values (unset, initial, and inherit), as well as none.

counter-reset, counter-increment

Forbids the global CSS values (unset, initial, and inherit), as well as none.

@counter-style, list-style-type

Forbids the global CSS values (unset, initial, and inherit), as well as the values:

  • none
  • inline
  • outside

Also, quite a few predefined values are implemented by the different browsers:

  • disc
  • circle
  • square
  • decimal
  • cjk-decimal
  • decimal-leading-zero
  • lower-roman
  • upper-roman
  • lower-greek
  • lower-alpha
  • lower-latin
  • upper-alpha
  • upper-latin
  • arabic-indic
  • armenian
  • bengali
  • cambodian
  • cjk-earthly-branch
  • cjk-heavenly-stem
  • cjk-ideographic
  • devanagari
  • ethiopic-numeric
  • georgian
  • gujarati
  • gurmukhi
  • hebrew
  • hiragana
  • hiragana-iroha
  • japanese-formal
  • japanese-informal
  • kannada
  • katakana
  • katakana-iroha
  • khmer
  • korean-hangul-formal
  • korean-hanja-formal
  • korean-hanja-informal
  • lao
  • lower-armenian
  • malayalam
  • mongolian
  • myanmar
  • oriya
  • persian
  • simp-chinese-formal
  • simp-chinese-informal
  • tamil
  • telugu
  • thai
  • tibetan
  • trad-chinese-formal
  • trad-chinese-informal
  • upper-armenian
  • disclosure-open
  • disclosure-close
grid-row-start, grid-row-end, grid-column-start, grid-column-end

Forbids the span value.


Forbids the global CSS values (unset, initial, and inherit), as well as the values will-change, auto, scroll-position, and contents.


Valid 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
_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

Invalid identifiers

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.
--toto            It must not start with two dashes. This would be a custom property.
'bilibob'         This would be a <string>.
"bilibob"         This would be a <string>.


CSS Values and Units Module Level 4
# custom-idents
CSS Will Change Module Level 1
# valdef-will-change-custom-ident
CSS Counter Styles Level 3
# typedef-counter-style-name
CSS Lists and Counters Module Level 3
# counter-properties

Browser compatibility

As this type is not a real type but a convenience type used to simplify the description of allowed values, there is no browser compatibility information as such.

See also