问题
I am trying to center align this button with flex box, but it is not working and I don't know what the problem is.
<html>
<head>
<style media="screen">
body {
display: flex;
align-items: center;
justify-content: center;
}
</style>
</head>
<body>
<button> Align Center</button>
</body>
</html>
来源：https://stackoverflow.com/questions/57113597/how-do-i-align-this-item-with-flex