code: function checkKeyframesSelectorsGroup(i) {
    const start = i;
    let l;
  
    if (l = checkKeyframesSelector(i)) i += l;
    else return 0;
  
    // Check for trailing space
    if (l = checkSC(i) && tokens[i].type !== TokenType.Newline) i += l;
  
    while (i < tokensLength) {
      const tempStart = i;
      let tempIndex = i;
      let tempLength;
  
      if (tempLength = checkDelim(tempIndex)) tempIndex += tempLength;
      else break;
  
      // Check for maxmimum space usage - 'space', '\n', 'space'
      if (tempLength = checkSC(tempIndex)) tempIndex += tempLength;
      if (tempLength = checkSC(tempIndex)) tempIndex += tempLength;
      if (tempLength = checkSC(tempIndex)) tempIndex += tempLength;
  
      if (tempLength = checkKeyframesSelector(tempIndex)) tempIndex += tempLength;
      else break;
  
      // Check for trailing space
      if (tempLength = checkSC(tempIndex) &&
          tokens[tempIndex].type !== TokenType.Newline) {
        tempIndex += tempLength;
      }
  
      i += tempIndex - tempStart;
    }
  
    tokens[start].selectorsGroupEnd = i;
  
    return i - start;
  }
docs: Check the keyframe's selector groups
  @param {number} i
  @return {number}