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<!DOCTYPE html>
<html>
<style> 
#myDIV {
  width: 220px;
  height: 300px;
  border: 1px solid black;
  display: flex;
}
</style>
<body>
<p>Click the "Try it" button to set the flex property of ALL DIV elements inside the "myDIV" element:</p>
<button onclick="myFunction()">Try it</button>
<div id="myDIV">
  <div style="background-color:coral;">RED</div>
  <div style="background-color:lightblue;">BLUE</div>  
  <div style="background-color:lightgreen;">Green div with more content.</div>
</div>
<script>
function myFunction() {
  const x = document.getElementById("myDIV");
  const y = x.getElementsByTagName("DIV");
  let i = 0;
  for (i; i < y.length; i++) {
    y[i].style.msFlex = "1"; // IE10
    y[i].style.flex = "1";
  }
}
</script>
</body>
</html>