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<!DOCTYPE html>
<html>
<head>
<style> 
#main {
  width: 350px;
  height: 100px;
  border: 1px solid #c3c3c3;
  display: flex;
}
#main div:nth-of-type(1) {flex-grow: 1;}
#main div:nth-of-type(2) {flex-grow: 1;}
#main div:nth-of-type(3) {flex-grow: 1;}
#myBlueDiv {
  animation: mymove 5s infinite;
}
@keyframes mymove {
  50% {flex-grow: 8;}
}
</style>
</head>
<body>
<h1>Animation of flex-grow</h1>
<p>Gradually change the flex-grow property of the "blue DIV" from 1 to 8, and back to 1:<p>
<div id="main">
  <div style="background-color:coral;"></div>
  <div style="background-color:lightblue;" id="myBlueDiv"></div>
  <div style="background-color:khaki;"></div>
</div>
</body>
</html>