一.利用flex布局，使元素在x轴与y轴上都居中
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>div中图片居中的方法</title>
<style>
. {
margin: 0px;
padding: 0px;
}
body {
width: 100%;
height: 100%;
margin: 0px;
}
#wrap {
width: 400px;
height: 400px;
background-color: red;
/*主要代码*/
display: flex;
justify-content: center;
align-items: center;
}
#picture{
width: 200px;
height: 200px;
background-color: black;
}
</style>
</head>
<body>
<div id="wrap">
<div id="picture"></div>
</div>
</body>
</html>