For many years the only device used to access webpages was a computer monitor. So designers created webpages with monitor sizes in mind. Concepts of safe table size grids, web safe colors, etc. were developed for optimal display on computer monitors.
However, all these norms have become irrelevant when people started browsing the web on the tablets and smartphones of various screen sizes. Hence it became impossible to design a webpage for a standard screen size. In viwe of this situation, a new way of creating adaptive page layouts with CSSthat will conform to the screen width of the users device.