Now a time has come, when the amount of mobile traffic accounts for a large majority 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 iPhones, smartphones, tablets, laptops, desktop PCs, smartTVs, etc. 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.
Sites meant for the desktop PCs built in the olden days look pathetic when viewed today through a small screen of an iPhone or a smartphone. The width of the pages cut off horizontally on the right side, and the viewer has to either keep scrolling horizontally (left to right and vice versa) to read the text or view the images, or has to zoom out to very small size, in which case the contents will fit the screen, but will be too small to view or read.
Look at the middle image of the three images below. This is how modern sites should work. The various sections (represented by four different colours) are aligned horizontally on a PC screen, whereas they automatically got vertically aligned when viewed through the smaller screens. One just has has to scroll down vertically (top to bottom) in the normal way to view everything, but not sidewise horizontally. Everything is within the width of the screen.
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.