code: function checkPseudoElement1(i) {
    const start = i;
    let l;
  
    // Skip `::`.
    i += 2;
  
    if (i >= tokensLength) return 0;
  
    if (l = checkIdent(i)) i += l;
    else return 0;
  
    if (i >= tokensLength ||
        tokens[i].type !== TokenType.LeftParenthesis) return 0;
  
    const right = tokens[i].right;
  
    // Skip `(`.
    i++;
  
    if (l = checkSC(i)) i += l;
  
    if (l = checkSelectorsGroup(i)) i += l;
    else return 0;
  
    if (l = checkSC(i)) i += l;
  
    if (i !== right) return 0;
  
    // Skip `)`.
    i++;
  
    return i - start;
  }
docs: (1) `::slotted(selector)`
  (2) `::slotted(selector, selector)`