Live Demo: Example of CSS3 flex-grow Property Animation

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Example of CSS3 flex-grow Property Animation</title>
<style>
.flex-container {
	width: 300px;
	height: 200px;
	font-size: 32px;
	border: 1px solid black;
	display: -webkit-flex; /* Safari */		
	display: flex; /* Standard syntax */
}
.flex-container div {
	-webkit-flex-grow: 1; /* Safari */
		-ms-flex-grow: 1; /* IE 10 */
			flex-grow: 1; /* Standard syntax */
}
.item1 {
	background: #e84d51;
}
.item2 {
	background: #7ed636;
}
.item3 {
	background: #2f97ff;
}
.animated {
    -webkit-animation: test 4s infinite; /* Chrome, Safari, Opera */
    animation: test 4s infinite;
}
/* Chrome, Safari, Opera */
@-webkit-keyframes test {
    50% {-webkit-flex-grow: 4;}
}
/* Standard syntax */
@keyframes test {
    50% {flex-grow: 4;}
}
</style>
</head>
<body>
	<p><strong>Warning:</strong> CSS Animations do not work in Internet Explorer 9 and earlier versions.</p>
	<p><strong>Note:</strong> The flex-grow property of the flex item2 is animating from its initial value "1" to "4", and back to the initial value "1" again up to infinite times.<p>
	<div class="flex-container">
        <div class="item1">1</div>
        <div class="item2 animated">2</div>
		<div class="item3">3</div>
    </div>
</body>
</html>