CSS identifiers are the labels used in property names, keyword values, and at-rule names, as well as in element type names, classes, and IDs within selectors. In the following example, fieldset, border, and none are all CSS identifiers:

fieldset {
  border: none;
}

According to the CSS2 specification, they can contain the characters a to z and 0 to 9, ISO 10646 characters (equivalent to Unicode) U+00A1 and higher, the hyphen (-), and the underscore (_); they can’t start with a digit, nor with a hyphen followed by a digit. Identifiers can also contain escaped characters.