You can check directly at the CSS grammar.
Basically 1, a name must begin with an underscore (
_), a hyphen (
-), or a letter(
z), followed by any number of hyphens, underscores, letters, or numbers. There is a catch: if the first character is a hyphen, the second character must2 be a letter or underscore, and the name must be at least 2 characters long.
In short, the previous rule translates to the following, extracted from the W3C spec.:
In CSS, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [a-z0-9] and ISO 10646 characters U+00A1 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit, or a hyphen followed by a digit. 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".
Identifiers beginning with a hyphen or underscore are typically reserved for browser-specific extensions, as in
1 It's all made a bit more complicated by the inclusion of escaped unicode characters (that no one really uses).
2 Note that, according to the grammar I linked, a rule starting with TWO hyphens, e.g.
--indent1, is invalid. However, I'm pretty sure I've seen this in practice.