现场演示：我的 HTML 文档

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Example of CSS border-right Property Animation</title>
<style>
.animated {
	width: 300px;
	padding: 40px 0;
	font: bold 46px sans-serif;
	text-align: center;
    background: #f5d8f4;	
    border: 2px solid orchid;
    -webkit-animation: test 4s infinite; /* Chrome, Safari, Opera */
    animation: test 4s infinite;
}
/* Chrome, Safari, Opera */
@-webkit-keyframes test {
    50% {border-right: 20px solid indigo;}
}
/* Standard syntax */
@keyframes test {
    50% {border-right: 20px solid indigo;}
}
</style>
</head>
<body>
	<p><strong>Warning:</strong> CSS Animations do not work in Internet Explorer 9 and earlier versions.</p>
	<p><strong>Note:</strong> Among all the individual border-right properties only the border-right-width, and border-right-color properties are animatable in CSS. The border-style property is not animatable. See the CSS <a href="https://www.appwang.com/css-reference/css-border-right-property.php" target="_top">border-right</a> shorthand property for details.<p>
	<div class="animated">Box</div>
</body>
</html>