How can I reorder my divs using only CSS?

Question

Given a template where the HTML cannot be modified because of other requirements, how is it possible to display (rearrange) a div above another div

Answer 1

I was looking for a way to change the orders of the divs only for mobile version so then I can style it nicely. Thanks to nickf reply I got to make this piece of code which worked well for what I wanted, so i though of sharing it with you guys:

//  changing the order of the sidebar so it goes after the content for mobile versions
jQuery(window).resize(function(){
    if ( jQuery(window).width() < 480 )
    {
        jQuery('#main-content').insertBefore('#sidebar');
    }
    if ( jQuery(window).width() > 480 )
    {
        jQuery('#sidebar').insertBefore('#main-content');
    }
    jQuery(window).height(); // New height
    jQuery(window).width(); // New width
});