Set min_height

min_contents.js

// #contents { min-height: }
function getBrowserHeight() {
    if ( window.innerHeight ) {
        return window.innerHeight;
    }
    else if ( document.documentElement && document.documentElement.clientHeight != 0 ) {
        return document.documentElement.clientHeight;
    }
    else if ( document.body ) {
        return document.body.clientHeight;
    }
    return 0;
}
 
var header = document.getElementsByTagName('header'),
    footer = document.getElementsByTagName('footer'),
    contents = document.getElementById('centents'),
    height_header = header[0].clientHeight,
    height_footer = footer[0].clientHeight,
    height_contents = getBrowserHeight() - height_header - height_footer;
    
contents.style.minHeight = height_contents+="px";