Now a time has come, when the amount of mobile traffic accounts for more than half of total internet traffic, and will grow further day by day. So, the modern approach to webpage designing uses a method which makes web pages render well on all kinds of devices and windows or screen sizes using smartphones, iPhones, tablets, laptops and desktop PCs. Depending upon the screen width, various elements of the page, like, structural panels, images, horizontal columns, charts, etc. automatically resize themselves or change their alignments from horizontal to vertical or vice versa.
If you are viewing a webpage through a laptop or a PC, you can test how responsive is its design by dragging the window to various sizes. If it's a perfectly designed program using the modern method, you will see how the view keeps changing as the page becomes narrower and narrower... and, in case you are using an updated Firefox browser on your laptop or PC, you can straightaway go to a smartphone view by pressing Ctrl + Shift + M. You can also drag it to a tablet size or rotate the view from portrait to landscape, the same way as you can do on a smartphone or an iPhone.