className

 
<!DOCTYPE HTML> 
<html> 
    <head> 
        <title>Example</title> 
        <style> 
            p { 
                border: medium double black; 
            } 
            p.newclass { 
                background-color: grey; 
                color: white; 
            } 
        </style> 
    </head> 
    <body> 
        <p id="textblock" class="Survey numbers"> 
            This is a test. This is a test.
            This is a test. This is a test.
            This is a test. This is a test.
        </p> 
        <button id="pressme">Press Me</button> 
        <script> 
            document.getElementById("pressme").onclick = function(e) { 
                document.getElementById("textblock").className += " newclass"; 
            }; 
        </script> 
    </body> 
</html>
Click to view the demo