Responsive web design is a design and technical paradigm for creating websites so that they can respond to the characteristics of the device being used, especially smartphones and tablet computers. Responsive websites are created using variable dimensions and the use of media queries.
Mobile First refers to a way of thinking in web design in which the display on mobile devices is given the highest priority in the development of a website.
Since the mobile Internet is on the rise and websites are increasingly accessed from small devices such as smartphones, a website should first be optimized for these same small devices. Until now, the focus in web design has been on optimizing the website for desktop screens.