css设置背景图片透明度

.demo{
	position: relative;
	width: 500px;
	height: 300px;
	line-height: 50px;
	text-align: center;
	color: red;
}
.demo::before{
	content: "";
	position: absolute;
	left: 0;
	top: 0;
	z-index: -1;  /*放在内容背后*/
	width: 500px;
	height: 300px;
	background: url(bg.png);
	background-size: cover;
	opacity: 0.5;  /*透明度*/
}
<div class="demo">图片半透明<br>内容不透明div>