    function set_background() {  
     docBody = document.getElementsByTagName("body")[0];  
      //Get all the p elements that are descendants of the body  
      myBodyElements = docBody.getElementsByTagName("p");  
    // get the first p elements  
      myp1 = myBodyElements[0];  
      myp1.style.background = "rgb(255,0,0)";  
    // get the second p elements  
      myp2 = myBodyElements[1];  
      myp2.style.background = "rgb(255,255,0)";  
    }  