使用filter属性来设置模糊值
效果：
css样式：
<style type="text/css">
.cover {
width: 600px;
height: 300px;
position: relative;
text-align: center;
line-height: 300px;
color: #fff;
margin: 20px auto;
}
.cover::before {
content: "";
position: absolute;
top: 0;
left: 0;
width: 600px;
height: 300px;
background: transparent url(Images/picture/5.jpg) center center no-repeat;
filter: blur(8px);
z-index: -1;
background-size: cover;
}
</style>
HTML结构：
<body>
<div class="cover">
<h2>背景模糊，文字清晰显示</h2>
</div>
</body>