code: function checkStylesheet(i) {
    const start = i;
    let l;
  
    // Check every token:
    while (i < tokensLength) {
      if (l = checkSC(i) ||
          checkRuleset(i) ||
          checkDeclaration(i) ||
          checkDeclDelim(i) ||
          checkAtrule(i) ||
          checkMixin(i)) i += l;
      else throwError(i);
    }
  
    return i - start;
  }
docs: Validate stylesheet: it should consist of any number (0 or more) of
  rulesets (sets of rules with selectors), @-rules, whitespaces or
  comments.
  @param {Number} i Token's index number
  @returns {Number}