well, its's not really a "demo", just a page i put together to fiddle with flexbox. view source to see how it works.
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nam consequat quam lorem, a imperdiet nulla commodo nec. Maecenas ut ligula.
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nam consequat quam lorem, a imperdiet nulla commodo nec. Maecenas ut ligula.
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nam consequat quam lorem, a imperdiet nulla commodo nec. Maecenas ut ligula.