code: function customIdent(token, addTokenToMatch) {
      if (token === null) {
          return false;
      }
  
      var identEnd = consumeIdentifier(token.value, 0);
      if (identEnd !== token.value.length) {
          return false;
      }
  
      var name = token.value.toLowerCase();
  
      // § 3.2. Author-defined Identifiers: the <custom-ident> type
      // The CSS-wide keywords are not valid <custom-ident>s
      if (name === 'unset' || name === 'initial' || name === 'inherit') {
          return false;
      }
  
      // The default keyword is reserved and is also not a valid <custom-ident>
      if (name === 'default') {
          return false;
      }
  
      // TODO: ignore property specific keywords (as described https://developer.mozilla.org/en-US/docs/Web/CSS/custom-ident)
  
      addTokenToMatch();
      return true;
  }
docs: https://developer.mozilla.org/en-US/docs/Web/CSS/custom-ident https://drafts.csswg.org/css-values-4/#identifier-value