code: function checkKeyframesBlocks(i) {
    if (i >= tokensLength) return 0;
  
    const blockEnd = tokens[i].block_end;
    const start = i;
    let l;
  
    if (!blockEnd) return 0;
  
    if (l = checkSC(i)) i += l;
  
    if (l = checkKeyframesBlock(i)) i += l;
  
    while (i < blockEnd) {
      if (l = checkSC(i)) i += l;
      else if (l = checkKeyframesBlock(i)) i += l;
      else if (l = checkAtrule(i)) i += l;
      else break;
    }
  
    if (i !== blockEnd + 1) return 0;
  
    return blockEnd + 1 - start;
  }
docs: Check all keyframe blocks - `5% {} 100% {}`
  @param {number} i
  @return {number}