code: function expandShorthandMap(styles) {
    var expanded = {};
    for (var key in styles) {
      var longhands = shorthandToLonghand[key] || [key];
      for (var i = 0; i < longhands.length; i++) {
        expanded[longhands[i]] = key;
      }
    }
    return expanded;
  }
docs: Given {color: 'red', overflow: 'hidden'} returns {
  color: 'color',
  overflowX: 'overflow',
  overflowY: 'overflow',
  }. This can be read as "the overflowY property was set by the overflow
  shorthand". That is, the values are the property that each was derived from.