Sys.Application.add_load(handleSmallWidth);

function handleSmallWidth() {
    if (screen.width <= 800) {
        var zone1 = $get('zone1');
        if (zone1 != null)
            zone1.style.display = 'none';
        var zone2 = $get('zone2');
        if (zone2 != null)
            zone2.style.display = 'none';
    }
}