code: function checkKeyframesSelector(i) {
    const start = i;
    let l;
  
    if (i >= tokensLength) return 0;
  
    if (l = checkIdent(i)) {
      // Valid selectors are only `from` and `to`.
      const selector = joinValues2(i, l);
      if (selector !== 'from' && selector !== 'to') return 0;
  
      i += l;
      tokens[start].keyframesSelectorType = 1;
    } else if (l = checkPercentage(i)) {
      i += l;
      tokens[start].keyframesSelectorType = 2;
    } else if (l = checkInterpolation(i)) {
      i += l;
      tokens[start].keyframesSelectorType = 3;
    } else {
      return 0;
    }
  
    return i - start;
  }
docs: Check a single keyframe selector - `5%`, `from` etc
  @param {Number} i
  @return {Number}