window.onload = setScreenClass; 
window.onresize = setScreenClass;

function setScreenClass(){
var fmt = screen.availWidth; 
//var fmt = document.documentElement.clientWidth;
var cls = (fmt<=0)?'screen_1024':(fmt>0&&fmt<=800)?'screen_800':(fmt>800&&fmt<=1024)?'screen_1024':(fmt>1024)?'screen_1280':'screen_1280';
document.getElementById('count').innerHTML=fmt+'px -> '+cls;
document.body.className=cls;
};