z-index
.box1{
    width: 200px;
    height: 200px;
    background-color:skyblue;
    position:relative;
    z-index:2;
}
.box2{
    width: 200px;
    height: 200px;
    background-color: red;
    position: absolute;
    left: 100px;
    top: 100px;
    z-index:3
}
.box3{
    width: 200px;
    height: 200px;
    background-color: black;
    position: relative;
    z-index:1;
}

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);
}

opacity只能设置0-1之间的数值
filter: alpha(opacity=50)兼容IE浏览器

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;
}
background-repeat:no-repeat 图片只显示一次
background-repeat:repeat-x 背景将在水平方向重复
background-repeat:repeat-y 背景将在垂直方向重复
background-repeat 设置背景重复
background-position 设置图片的位置:
top lefttop center top right center left center center center right
background-attachment 设置图片 fixed 固定