code: function checkKeyframesBlock(i) {
    const start = i;
    let l;
  
    if (i >= tokensLength) return 0;
  
    if (l = checkKeyframesSelectorsGroup(i)) i += l;
    else return 0;
  
    if (l = checkSC(i)) i += l;
  
    if (l = checkBlock(i)) i += l;
    else return 0;
  
    return i - start;
  }
docs: Check a single keyframe block - `5% {}`
  @param {number} i
  @return {number}