It’s not just web design. It’s “Responsive” web design.
Through the use of an extensive grid system, a website can be created in order to accommodate to whatever dimensions are being used to view the website without having to load separate pages through the use of a device detection system.
Responsive grids are capable of adjusting the page that you are already on based on the browser dimensions that you are using no matter what device or browser that you are currently using. So instead of redirecting to a different version of your website, the user stays on the current page and simply renders the content differently in order to make the content viewable just the same but with simple modifications that make it look clean and viewable without having to scroll right and left or zoom in and out.
Suffice it to say, responsive grid systems do not work alone. These systems require very special conditional CSS3 coding in order to accommodate the various browser dimensions. This allows the display to work properly while rendering or disabling the rendering of various elements and other properties as well. For example, a standard horizontal menu can turn into a mobile menu simply by resizing the browser. And this is just to say the least about responsive web design.