.box1{
width: 200px;
height: 200px;
background-color: #0bcd96;
position:relative;
z-index:3;
}
.box2{
width: 200px;
height: 200px;
background-color: #1e389a;
position: absolute;
left: 100px;
top: 100px;
}
.box3{
width: 200px;
height: 200px;
background-color: #6a1752;
position: relative;
z-index:2;
}
如果没有开启定位的元素或者是定位为默认值，会忽略z-index的声明
父元素的层级再怎么高也不会盖住子元素，可以理解为（水涨船高）
opacity（透明）
.box1{
width: 200px;
height: 200px;
background-color: #0bcd96;
position:relative;
opacity: 50%;
filter: alpha(opacity=50);
}
.box2{
width: 200px;
height: 200px;
background-color: #1e389a;
position: absolute;
left: 100px;
top: 100px;
opacity:0.5px；
filter: alpha(opacity=50);
}
.box3{
width: 200px;
height: 200px;
background-color: #6a1752;
position: relative;
opacity: 50%;
filter: alpha(opacity=50);
}
背景（background-image）
.box1{
width: 500px;
height: 500px;
margin:0 auto;
background-color: #0bcd96;
background-image: url(../css/logo_new@2x.png);
background-repeat:no-repeat;
}
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