通过css控制，可以实现加载网络图片时，未加载完成的时候显示本地一张占位图，加载完成后显示网络图片；
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
img {
position: relative;
}
img::after {
content: "";
height: 100%;
width: 100%;
position: absolute;
left: 0;
top: 0;
background: url(iphonex.png ) no-repeat center;
}
</style>
</head>
<body>
<img src="">
</body>
<script>
setTimeout(function() {
document.querySelectorAll("img")[0].src = ‘//upload-images.jianshu.io/upload_images/7450593-65067eb4cf76d882.jpg?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240‘;
}, 3000);
</script>
</html>
原文：https://www.cnblogs.com/010101-/p/10649026.html