Solution: How to change the background-color if the viewport is 400 pixels wider?

w3resource Tutorial

Resize the browser window and see @media effect!

CSS, stands for Cascading Style Sheet is a computer language to describe presentation (for example width, height, color, background color, alignment etc.) of HTML and XML (and XML based languages like XHTML, SVG) web documents. In all the examples of our tutorials, we have used HTML for implementing CSS.