Let’s take a simple example where a web page displays three images alongside each other. Responsive web design means that the web pages respond to the size of the display screen. CSS 2.12 introduced media types, but it was not until CSS 3 that true flexibility, or responsiveness, could be achieved through style sheets. That led to the initiative to provide a means of flexible web design that could adapt to the display. It was also clear that new technology would push screen resolutions up further. At the latter resolution, websites designed for 640×480 did not look so good. This was followed by XGA, with resolutions of 1024×760. When SVGA became available, screen resolutions increased to 800×600. Designers of the first websites could happily construct pages that displayed optimally at these dimensions. When the web first became available to the world at large, almost every user had a simple VGA screen that had a resolution of 640×480 pixels. Responsive web design is all about having flexible web pages that adjust according to the size of the screen on which they are displayed. Responsive web design means that the web pages respond to the size of the display screen… Responsive Web Design Thus, responsive web design may not be a mobile-first design, but a mobile-first design will almost certainly be responsive web design. Responsive web design is a design implementation or style, while mobile-first is a design methodology or criterion. Understanding these differences is essential if you are talking to a web designer or design company about what you want for your website.