If one had to describe RWD in a sentence, then responsive design describes how the content is displayed across various screens and devices such as mobiles, tablets, phablets, or desktops. To understand what this means, let's use water as an example. The property of water is that it takes the shape of the container in which it is poured. It is an approach in which a website or a webpage adjusts its layout according to the size or resolution of the screen dynamically. This ensures that users get the best experience while using the website.

We develop a single website that uses a single code base. This will contain fluid, flexible images, proportion-based grids, fluid images, or videos and CSS3 media queries to work across multiple devices and device resolutions. The key to making them work is the use of percentage values, in place of fixed units such as pixels or ems-based sizes.

The best part of this is that we can use this technique without the knowledge or need of server based/backend solutions; to see it in action, we can use Packt's website as an example. Go ahead and browse https://www.packtpub.com/web-development/mastering-html5-forms; this is what we will see as a desktop view:

The mobile view for the same website shows this, if viewed on a smaller device:

We can clearly see the same core content is being displayed (that is, an image of the book, the buy button, pricing details, and information about the book), but elements such as the menu have been transformed into a single drop-down located in the top-left corner. This is what RWD is all about—producing a flexible design that adapts according to which device we choose to use, in a format that suits the device being used.

Let's try it with another technology site; I am sure some of you are familiar with the A List Apart site (hosted at http://alistapart.com and founded by the well-known Jeffery Zeldman):

Try resizing your browser window. This is a perfect example of how a simple text site can reflow content with minimal changes; in place of the text menu, we now have the hamburger icon (which we will cover later in this chapter):

While the text in this site realigns with minimal difficulty, it seems that the top image doesn't work so well—it hasn't resized as part of the change, so will appear cut off on smaller devices.

Although this may seem a complex task to achieve, in reality it boils down to following some simple principles; how these are implemented will ultimately determine the success of our site. We've seen some in use as part of viewing Packt's and A List Apart's sites at various sizes—let's take a moment to explore the principles of how responsive design works, and why it is an important part of creating a site for the modern Internet.