效果图：
完整代码：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8" />
<title>扩展方块加载动画</title>
<style type="text/css">
body {
background: #ECF0F1;
display: flex;
justify-content: center;
align-items: center;
height: 100vh;
margin: 0;
}
.expanding-boxes {
display: flex;
justify-content: space-between;
align-items: center;
width: 60px;
}
.expanding-boxes div {
width: 10px;
height: 10px;
background-color: #3498db;
animation: expand 0.6s infinite alternate;
}
.expandin
