In CSS 2.1, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [A-Za-z0-9] and ISO 10646 characters U+00A1 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit. Only properties, values, units, pseudo-classes, pseudo-elements, and at-rules may start with a hyphen (-); other identifiers (e.g. element names, classes, or IDs) may not. Identifiers 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".