Which characters are valid in CSS class names/selectors?
What characters/symbols are allowed within CSS class selectors? I know that the following characters are invalid, but what characters are valid?
~ ! @ $ % ^ & * ( ) + = , . / ' ; : " ? > < [ ] { } | ` #
You can check directly at the CSS grammar.
Basically1, a name must begin with an underscore ( _ ), a hyphen ( - ), or a letter( a – 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.
-?[_a-zA-Z]+[_a-zA-Z0-9-]*
Identifiers beginning with a hyphen or underscore are typically reserved for browser-specific extensions, as in -moz-opacity .
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, eg --indent1 , is invalid. However, I'm pretty sure I've seen this in practice.
To my surprise most answers here are wrong. It turns out that:
Any character except NUL is allowed in CSS class names in CSS. (If CSS contains NUL (escaped or not), the result is undefined. [CSS-characters])
Mathias Bynens' answer links to explanation and demos showing how to use these names. Written down in CSS code, a class name may need escaping, but that doesn't change the class name. Eg an unnecessarily over-escaped representation will look different from other representations of that name, but it still refers to the same class name.
Most other (programming) languages don't have that concept of escaping variable names (“identifiers”), so all representations of a variable have to look the same. This is not the case in CSS.
Note that in HTML there is no way to include space characters (space, tab, line feed, form feed and carriage return) in a class name attribute, because they already separate classes from each other.
So, if you need to turn a random string into a CSS class name: take care of NUL and space, and escape (accordingly for CSS or HTML). Done.
Read the W3C spec. (this is CSS 2.1, find the appropriate version for your assumption of browsers)
edit: relevant paragraph follows:
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 "B26 W3F".
edit 2: as @mipadi points out in Triptych's answer, there's this caveat, also in the same webpage:
In CSS, identifiers may begin with '-' (dash) or '_' (underscore). Keywords and property names beginning with '-' or '_' are reserved for vendor-specific extensions. Such vendor-specific extensions should have one of the following formats:
'-' + vendor identifier + '-' + meaningful name
'_' + vendor identifier + '-' + meaningful name
Example(s):
For example, if XYZ organization added a property to describe the color of the border on the East side of the display, they might call it -xyz-border-east-color.
Other known examples:
-moz-box-sizing
-moz-border-radius
-wap-accesskey
An initial dash or underscore is guaranteed never to be used in a property or keyword by any current or future level of CSS. Thus typical CSS implementations may not recognize such properties and may ignore them according to the rules for handling parsing errors. However, because the initial dash or underscore is part of the grammar, CSS 2.1 implementers should always be able to use a CSS-conforming parser, whether or not they support any vendor-specific extensions.
Authors should avoid vendor-specific extensions
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