<STYLE TYPE="text/css" > 
.redText {color: red;} 
#pid {color: red;} 
</STYLE > 
<P id="redText" CLASS="redText" >11 </p > 
<P id="pid" >22 </p > 
<input type="button" onclick="test()" value=" test " />
<script language="javascript" type="text/javascript" > 
function test()
{
    var ss = document.styleSheets[0];
    var rules = ss.rules?ss.rules:ss.cssRules
    for(var i=0;i<rules.length;i++)
    {
        if(rules[i].selectorText=="#pid")
        {
            rules[i].style.color="yellow";
        }
    }
    
    
}
</script>