现场演示：我的 HTML 文档

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Example of CSS3 flex 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: 1; /* Safari */
	    -ms-flex: 1; /* IE 10 */
	        flex: 1; /* Standard syntax */
}
.item1 {
	background: #e84d51;
}
.item2 {
	background: #7ed636;
}
.animated {
    -webkit-animation: test 4s infinite; /* Chrome, Safari, Opera */
    animation: test 4s infinite;
}
/* Chrome, Safari, Opera */
@-webkit-keyframes test {
    50% {-webkit-flex: 3;}
}
/* Standard syntax */
@keyframes test {
    50% {flex: 3;}
}
</style>
</head>
<body>
	<p><strong>Warning:</strong> CSS Animations do not work in Internet Explorer 9 and earlier versions.</p>
	<p><strong>Note:</strong> All the individual columns properties i.e. flex-grow, flex-shrink and flex-basis are animatable in CSS. See the CSS <a href="https://www.appwang.com/css-reference/css3-flex-property.php" target="_top">flex</a> shorthand property for details.<p>
	<div class="flex-container">
        <div class="item1">1</div>
        <div class="item2 animated">2</div>
    </div>
</body>
</html>

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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Example of CSS3 flex 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: 1; /* Safari */
        -ms-flex: 1; /* IE 10 */
            flex: 1; /* Standard syntax */
}
.item1 {
    background: #e84d51;
}
.item2 {
    background: #7ed636;
}
.animated {
    -webkit-animation: test 4s infinite; /* Chrome, Safari, Opera */
    animation: test 4s infinite;
}
/* Chrome, Safari, Opera */
@-webkit-keyframes test {
    50% {-webkit-flex: 3;}
}
/* Standard syntax */
@keyframes test {
    50% {flex: 3;}
}
</style>
</head>
<body>
    <p><strong>Warning:</strong> CSS Animations do not work in Internet Explorer 9 and earlier versions.</p>
    <p><strong>Note:</strong> All the individual columns properties i.e. flex-grow, flex-shrink and flex-basis are animatable in CSS. See the CSS <a href="https://www.appwang.com/css-reference/css3-flex-property.php" target="_top">flex</a> shorthand property for details.<p>