やり方
横幅の大きさを指定します。横幅の大きさを指定しないと中央寄せできません。marginはautoにします。すると中央に要素を配置できます。
.content {
max-width: 500px;
margin: auto;
}
ソースコード
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
body {
background: #555;
}
.content {
max-width: 500px;
margin: auto;
background: white;
padding: 10px;
}
</style>
</head>
<body>
<div class="content">
<h1>This page is horizontally centered on screens that are wider than 500 pixels.</h1>
<h1>Resize the browser window to see the effect.</h1>
<p>This page is always centered on screens that are wider than 500px. On screens that are less than
500px wide, it will span the whole of the page.</p>
<p>Content inside this container is centered horizontally.</p>
<p>Content inside this container is centered horizontally.</p>
<p>Content inside this container is centered horizontally.</p>
<p>Content inside this container is centered horizontally.</p>
<p>Content inside this container is centered horizontally.</p>
<p>Content inside this container is centered horizontally.</p>
<p>Content inside this container is centered horizontally.</p>
<p>Content inside this container is centered horizontally.</p>
<p>Content inside this container is centered horizontally.</p>
</div>
</body>
</html>
参考サイト
https://www.w3schools.com/howto/howto_css_center_website.asp
https://www.nishi2002.com/16106.html