Responsive, flexible designs can make for complicated resizing -- after all there are a lot of elements on a page and scaling them all for different screen sizes isn't easy. But there's another ...

I just discovered a free tool called Mobirise for building mobile and responsive Web sites that is, I think, quite excellent and definitely worth checking out.

For example, creating a responsive HTML5/CSS landing page will help you understand the principles of responsive design, ensuring that your web pages look great on any device.