您可以在相对定位的元素内有一个绝对定位的元素：

<div id="container">
    <div id="background">
    Text to have as background
    </div>
    Normal contents
</div>

然后是CSS：

#container {
   position: relative;
}

#background {
   position: absolute;
   top: 0;
   left: 0;
   bottom: 0;
   right: 0;
   z-index: -1;
   overflow: hidden;
}