中心对准一个绝对定位的Div

div#thing {
  position: absolute;
  top: 0px;
  z-index: 2;
  margin: 0 auto;
}

<div id="thing">
   <p>text text text with no fixed size, variable font</p>
</div>
div#thing {
  position: absolute;
  top: 0px;