Ethan was one of the first guys to talk about responsive web design publicly on a dense level back in 2010 within this article.
It defines how response web design is at the forefront of how we are going to be viewing information on the web, and how it will simplify the design process as well as create a more even user experience across multiple devices.
He goes very deep into the idea of media queries, and how we can use them within CSS and not within the HTML in order to focus CSS style functions at a specific device width. this creates much more semantic HTML content as we are not using it to vary the style, and we do not need to create multiple CSS files and call them in the head tag whenever a specific device width is reached.
brilliant introduction into responsive web design and the development on the test site was a good help