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Is "--" a valid CSS3 identifier?

According the CSS Level 3 specification, for parsing the start of an identifier, you:

Check if three code points would start an identifier

Look at the first code point:

  • If the first character is -, then we have a valid identifier if:
    1. The second code point is an identifier-start code point ([a-zA-Z_] or non-ASCII).
    2. The second code point is -.
    3. The second and third character form a valid escape.

Otherwise, we do not have a valid identifier start. After determining if we have a valid identifier start, the only requirements to have a valid <ident-token> is we have 0 or more of any combination of the following:

  1. Escape tokens
  2. ASCII letters
  3. Digits
  4. _ or -
  5. Non-ASCII characters

Since we do not require any characters following an identifier start token, this would suggest that -- is a valid identifier, even if never supported by any browser or framework. However, even official CSS validation services (maintained by those that design the CSS specifications) do not consider this a valid identifier. Is this merely a bug in the validation service?

Solution

  • The -- custom property identifier is reserved for future use, but current browsers incorrectly treat it as a valid custom property.

    See also