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HTML

            
              
            
          
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↑ Insert the most common viewport meta tag

CSS

            
              body {
    background: blue;
    transition: 1s all ease;
}
@media (max-width: 700px) {
  body {  
    background: red;
  }
}
@media (min-width: 800px) {
  body {  
    background: green;
  }
}
            
          
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JS

            
              
            
          
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