code: function checkKeyframesRule(i) {
    const start = i;
    let l;
  
    if (i >= tokensLength) return 0;
  
    if (l = checkAtkeyword(i)) i += l;
    else return 0;
  
    const atruleName = joinValues2(i - l, l);
    if (atruleName.toLowerCase().indexOf('keyframes') === -1) return 0;
  
    if (l = checkSC(i)) i += l;
    else return 0;
  
    if (l = checkIdentOrInterpolation(i) || checkPseudoc(i)) i += l;
    else return 0;
  
    if (l = checkSC(i)) i += l;
  
    if (l = checkKeyframesBlocks(i)) i += l;
    else return 0;
  
    return i - start;
  }
docs: Check if token is part of a @keyframes rule.
  @param {number} i Token's index number
  @return {number} Length of the @keyframes rule