<h1 id="size">window size</h1>
body{width: 100%; border: 10px solid red; margin: 0 auto;}
h1{text-align: center;}
var body = document.getElementsByTagName("body")[0];
window.onresize = function(event){
  var innerWidth = window.innerWidth;
  innerWidth <= "1280" ? body.style.borderColor = "blue" : body.style.borderColor = "black";
}

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