效果如下：
代码如下：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.container{
width: 350px;
/* flex布局 */
display: flex;
/* 水平对齐方式 */
justify-content: space-evenly;
/* 实现换行 */
flex-wrap: wrap;
border:2px solid black;
padding-bottom: 10px;
}
.item{
width: 100px;
height: 100px;
background-color: #62e03b;
margin-top: 10px;
}
</style>
</head>
<body>
<div class="container">
<div class="item">1</div>
<div class="item">2</div>
<div class="item">3</div>
<div class="item">4</div>
<div class="item">5</div>
<div class="item">6</div>
<div class="item">7</div>
<div class="item">8</div>
<div class="item">9</div>
</div>
</body>
</html>扩展：若想让每个格子的内容呈现以下效果，则在每个格子里实现flex布局
.item{
width: 100px;
height: 100px;
background-color: #62e03b;
margin-top: 10px;
/* 以下为新加的 */
display: flex;
justify-content: center;
align-items: center;
}