function screen_width(){	
	var myWidth = 0, myHeight = 0;
	if( typeof( window.innerWidth ) == 'number' ) {//Non-IE
		myWidth = window.innerWidth;
		myHeight = window.innerHeight;
	} 
	else if( document.documentElement && ( document.documentElement.clientWidth || document.documentElement.clientHeight ) ) {//IE 6+ in 'standards compliant mode'
		myWidth = document.documentElement.clientWidth;
		myHeight = document.documentElement.clientHeight;
	} 
	else if( document.body && ( document.body.clientWidth || document.body.clientHeight ) ) {//IE 4 compatible
		myWidth = document.body.clientWidth;
		myHeight = document.body.clientHeight;
	}
	return myWidth;
}
function set_left() {
	document.getElementById('div_contenuto_pagina').style.left = (screen_width()-980)/2 + "px";
	//console.log(document.getElementById('div_contenuto_pagina').style);
}
window.onresize = set_left;
window.setInterval("set_left()", 100);