How to stretch div height to fill parent div - CSS

Answer 1

I'd solve it with a javascript solution (jQUery) if the sizes can vary.

window.setTimeout(function () {
    $(document).ready(function () {
        var ResizeTarget = $('#B');

        ResizeTarget.resize(function () {
            var target = $('#B2');
            target.css('height', ResizeTarget.height());
        }).trigger('resize');
    }); 
}, 500);