要为元素的width和height属性添加过渡效果，您可以尝试运行以下代码
<!DOCTYPE html>
<html>
<head>
<style>
div {
width: 150px;
height: 150px;
background: blue;
transition: width 3s;
}
div:hover {
width: 250px;
height: 250px;
}
</style>
</head>
<body>
<h1>Heading One</h1>
<p>Hover over the below box to change its width and height.</p>
<div></div>
</body>
</html>