var h_main = $('main').clientHeight;
var h_sub = $('sub').clientHeight;
var h_header = $('header').clientHeight;
var h_footer = $('footer').clientHeight;

window.onload = resize;
window.onresize = resize;

function resize() {
    var h_body = document.body.clientHeight;
    var h_content = Math.max(h_main,h_sub,h_body - h_header - h_footer)
    $('sub').style.height = $('main').style.height = h_content + 'px';
}