code: function setCss(el, prop, val, idx) {
    if ('string' == typeof prop) {
      var styles = getCss(el);
      if (typeof val === 'function') {
        val = val.call(el, idx, styles[prop]);
      }
  
      if (val === '') {
        delete styles[prop];
      } else if (val != null) {
        styles[prop] = val;
      }
  
      el.attribs.style = stringify(styles);
    } else if ('object' == typeof prop) {
      Object.keys(prop).forEach(function(k){
        setCss(el, k, prop[k]);
      });
    }
  }
docs: Set styles of all elements.
  
  @param {String|Object} prop
  @param {String} val
  @param {Number} idx - optional index within the selection
  @return {self}
  @api private