You can have an absolutely positioned element inside of your relative positioned element

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

css

#container {
   position: relative;
}

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

https://stackoverflow.com/questions/1191464/is-there-a-way-to-use-use-text-as-the-background-with-css