在Bootstrap中以不同屏幕尺寸在中心对齐Flex项目
要在不同屏幕尺寸的中心对齐项目，请使用justify-content-*-center类。
对于较小的屏幕尺寸，对齐后的弹性项目如下所示：
<div class="d-flex justify-content-sm-center bg-warning mb-5"> <div class="p-2 bg-danger">GOLF</div> <div class="p-2 bg-info">CRICKET</div> <div class="p-2 bg-secondary">SQUASH</div> </div>
对于中等尺寸的屏幕，对齐后的弹性项目如下所示：
<div class="d-flex justify-content-md-center bg-warning mb-5"> <div class="p-2 bg-danger">GOLF</div> <div class="p-2 bg-info">CRICKET</div> <div class="p-2 bg-secondary">SQUASH</div> </div>
对于大屏幕尺寸，对齐后的弹性项目如下所示：
<div class="d-flex justify-content-lg-center bg-warning mb-5"> <div class="p-2 bg-danger">GOLF</div> <div class="p-2 bg-info">CRICKET</div> <div class="p-2 bg-secondary">SQUASH</div> </div>
让我们看看如何实现justify-content-*-center-
示例
<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap Example</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="https://cdn.staticfile.org/jquery/3.3.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container mt-3">
<h3>Sports</h3>
<h4>Centered on small screen</h4>
<div class="d-flex justify-content-sm-center bg-warning mb-5">
<div class="p-2 bg-danger">GOLF</div>
<div class="p-2 bg-info">CRICKET</div>
<div class="p-2 bg-secondary">SQUASH</div>
</div>
<h4>Centered on medium screen</h4>
<div class="d-flex justify-content-md-center bg-warning mb-5">
<div class="p-2 bg-danger">GOLF</div>
<div class="p-2 bg-info">CRICKET</div>
<div class="p-2 bg-secondary">SQUASH</div>
</div>
<h4>Centered on large screen</h4>
<div class="d-flex justify-content-lg-center bg-warning mb-5">
<div class="p-2 bg-danger">GOLF</div>
<div class="p-2 bg-info">CRICKET</div>
<div class="p-2 bg-secondary">SQUASH</div>
</div>
</div>
</body>
</html>