how to make responsive website in css less than screen size code example

Example 1: css media query

/* BOOSTRAP MEDIA BREAKPOINTS */
/* Small devices (landscape phones, 576px and up) */
@media (min-width: 576px) {  
  .selector {
  	background-color:#f00;
  }
}
/* Medium devices (tablets, 768px and up) The navbar toggle appears at this breakpoint */
@media (min-width: 768px) {}
/* Large devices (desktops, 992px and up) */
@media (min-width: 992px) {}
/* Extra large devices (large desktops, 1200px and up) */
@media (min-width: 1200px) {}

Example 2: media query css for all devices

@media (min-width: 600px) and (max-width: 800px) {

/* your css code here  */

/* html { background: red; } */


}