css media query

CSS
/* 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) {    
}@media only screen and (max-width: 600px) {
  .class {
    // Your CSS
  }
}CSS @media Rule

Example
Change the background color of the <body> element to "lightblue" when the browser window is 600px wide or less:

Syntax
@media only screen and (max-width: 600px) {
  body {
    background-color: lightblue;

    }
}

 @media only screen and (max-width: 600px) {
  body {

   
background-color: lightblue;
  }
}
Source

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