CSS：根据容器宽度调整字母间的间距？

span{
  -webkit-transform: rotate(-90deg);
  -o-transform: rotate(-90deg);
  -moz-transform: rotate(-90deg);
  filter: progid:DXImageTransform.Microsoft.BasicImage(rotation=3);
  height: 100%;
}

.container{
  width: 40px;
  height: 500px;  /* <- this can change */
}

$(".container").each(function(idx, el) {
    $(el).css("letter-spacing", calculateSpacing(el, $(el).height()));        
});