Set media query in CSS

@media screen and (min-width: 900px) {
  article {
    width: 600px;
  }
}

The @media CSS at-rule can be used to apply part of a style sheet based on the result of one or more media queries. With it, you specify a media query and a block of CSS to apply to the document if and only if the media query matches the device on which the content is being used.