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13-[CSS]-postion位置：相relative，绝absolute，固fixed，static（默认）,z-index

1、postion位置属性

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style type="text/css">
        .box1{
            width: 200px;
            height: 200px;
            background-color: red;
            /*如果对当前元素仅仅设置相对定位，那么与标准流下的盒子没有什么区别*/
            position: relative;
            /*设置相对定位 我们就可以使用四个方向的属性  top left right bottom

            相对定位：相对于自己原来的本身定位 top:20px; 那么盒子相对于原来的位置向下移动。相对定位仅仅的微调我们元素的位置
            */
            top: 20px;
            left: 30px;
        }
    </style>
</head>
<body>

    <!-- 定位有三种： 1.相对定位 2.绝对定位 3.固定定位
        这三种定位，每种定位都暗藏玄机，所以我们要一一单讲

        position:relative;
        position:absolute;
        position:fixed;

     -->

     <div class="box1"></div>
 
</body>
</html>

2、相对定位relative

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style type="text/css">
        
        *{
            padding: 0;
            margin: 0;
        }
        div{
            width: 200px;
            height: 200px;

        }
        .box1{
            background-color: red;
        }
        .box2{
            background-color: green;
            position: relative;
            top: 50px;
            left: 100px;
        }
        .box3{
            background-color: blue;
        }


    </style>
</head>
<body>

    <div class="box1"></div>
    <div class="box2"></div>
    <div class="box3"></div>

    
</body>
</html>

　　（2）相对定位有bug

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>超链接美化</title>
    <style type="text/css">
        
        *{
            padding: 0;
            margin: 0;
        }
        ul{
            list-style: none;
        }
        .nav{
            width: 960px;
            /*height: 40px;*/
            overflow: hidden;
            margin: 100px auto ;
            background-color: purple;
            /*设置圆角*/
            border-radius: 5px;
        }
        .nav ul li{
            float: left;
            width: 160px;
            height: 40px;
            line-height: 40px;
            text-align: center;
        }
        .nav ul li.xiaoming{
            position: relative;
            top: 40px;
            left: 30px;
        }
        .nav ul li a{
            display: block;
            width: 160px;
            height: 40px;
            color: white;
            font-size: 20px;
            text-decoration: none;
            font-family: 'Hanzipen SC';
        }
        /*a标签除外，不继承父元素的color*/


        .nav ul li a:hover{
            background-color: red;
            font-size: 22px;
        }
    </style>
</head>
<body>
    
    <div class="nav">
        <ul>
            <li>
                <a href="">网站导航</a>
            </li>
            <li class="xiaoming">
                <a href="">网站导航</a>
            </li>
            <li>
                <a href="">网站导航</a>
            </li>
            <li>
                <a href="">网站导航</a>
            </li>
            <li>
                <a href="">网站导航</a>
            </li>
            <li>
                <a href="">网站导航</a>
            </li>
        </ul>
    </div>
</body>
</html>

　　（3）微调元素

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style type="text/css">
        *{
            padding: 0;
            margin: 0;
        }
        div{
            margin: 100px;
        }
        .user{
            font-size: 25px; 
        }
        .btn{
            position: relative;
            top: 3px;
            left: -5px;
        }

    </style>
</head>
<body>
    <!-- 微调我们元素位置-->

    <div>
        
        <input type="text" name="username"   class="user">
        <input type="button" name="" value="点我" class="btn">
    </div>
    
</body>
</html>

3、绝对定位absolute

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style type="text/css">
        
        *{
            padding: 0;
            margin: 0;
        }
        div{
            width: 200px;
            height: 200px;

        }
        .box1{
            background-color: red;
            
            position: absolute;
            top:10px;
            left:10px;
        }
        .box2{
            background-color: green;
            
        }
        .box3{
            background-color: blue;
        }
        span{
            width: 100px;
            height: 100px;
            background-color: pink;
            position: absolute;
        }


    </style>
</head>
<body>

    

    <div class="box1"></div>
    <div class="box2"></div>
    <div class="box3"></div>
    <span>文字</span>
    
</body>
</html>

　　（1）绝对定位参考点

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style type="text/css">
        body{
            width: 100%;
            height: 2000px;
            border: 10px solid green;
        }
        
        .box{
            width: 200px;
            height: 200px;
            background-color: red;

            position: absolute;
            bottom: 100px;
            left: 18px;
        }
    </style>
</head>
<body>
    <div class="box">
        
    </div>


    
</body>
</html>

　　（2）以父辈当做参考点

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style type="text/css">
        *{
            padding: 0;
            margin: 0;
        }
        .box{
            width: 300px;
            height: 300px;
            border: 5px solid red;
            margin: 100px;

            /*父盒子设置相对定位*/
            position: relative;
            padding: 50px;
        }
        .box2{
            width: 300px;
            height: 300px;
            background-color: green;
            position: relative;
            
        }

        .box p{
            width: 100px;
            height: 100px;
            background-color: pink;
            /*子元素设置了绝对定位*/
            position: absolute;
            top: 0;
            left: 0;
        }

    </style>
</head>
<body>
    <div class="box">

        <div class="box2">
            <p></p>
        </div>
    </div>
    
</body>
</html>

　　（3）父相子绝，父绝子绝，父固子绝

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style type="text/css">
        *{
            padding: 0;
            margin: 0;
        }
        .box{
            width: 300px;
            height: 300px;
            border: 5px solid red;
            margin: 100px;
            /*父盒子设置相对定位*/
            position: absolute;
            padding: 50px;
        }
        

        .box p{
            width: 100px;
            height: 100px;
            background-color: pink;
            /*子元素设置了绝对定位*/
            position: absolute;
            top: 10px;
            left: 20px;
        }


    </style>
</head>
<body>

    <div class="box">

            <p></p>

    </div>
    
</body>
</html>

　　（4）绝对定位的盒子无视父辈的 padding

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style type="text/css">
        *{
            padding: 0;
            margin: 0;
        }
        .father{
            width: 300px;
            height: 300px;
            margin: 100px;
            border: 10px solid red;
            position: relative;
            padding: 50px;
        }
        .father p{
            width: 100px;
            height: 100px;
            background-color: yellow;
            position: absolute;
            top: 10px;
            left: 40px;
        }
    </style>
</head>
<body>
    <div class="father">
        <p></p>
    </div>
    
</body>
</html>

　　（5）绝对定位盒子居中

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style type="text/css">
        *{
            padding: 0;
            margin: 0;
        }
        .box{
            width: 100%;
            height: 69px;
            background: #000;
        }
        .box .c{
            width: 960px;
            height: 69px;
            background-color: pink;
            /*margin: 0 auto;*/
            position: relative;
            left: 50%;
            margin-left: -480px;

            /*设置绝对定位之后，margin:0 auto;不起任何作用，
            如果想让绝对定位的盒子居中。当做公式记下来 
                设置子元素绝对定位，
                然后left:50%;
                margin-left等于元素宽度的一半，
            实现绝对定位盒子居中*/
        }


    </style>
</head>
<body>
    <div class="box">
        <div class="c"></div>
    </div>
    
</body>
</html>

4、fixed固定属性

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style type="text/css">
        
        *{
            padding: 0;
            margin: 0;
        }
        p{
            width: 100px;
            height: 100px;
            background-color: red;
            position: fixed;
            bottom: 30px;
            right: 40px;
        }
    </style>
</head>
<body>
    
    <div>
        <p></p>
        <img src="./bojie.jpg" alt="">
        <img src="./bojie.jpg" alt="">
        <img src="./bojie.jpg" alt="">
        <img src="./bojie.jpg" alt="">
        <img src="./bojie.jpg" alt="">
        <img src="./bojie.jpg" alt="">

    </div>
</body>
</html>

　　（1）返回顶部

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style type="text/css">
        
        *{
            padding: 0;
            margin: 0;
        }
        p{
            width: 100px;
            height: 100px;
            background-color: red;
            position: fixed;
            bottom: 30px;
            right: 40px;
            line-height: 100px;
            font-size: 20px;
            text-align: center;
            color: #fff;
        }
    </style>
</head>
<body>
    
    <div>
        <p>返回顶部</p>
        <img src="./bojie.jpg" alt="">
        <img src="./bojie.jpg" alt="">
        <img src="./bojie.jpg" alt="">
        <img src="./bojie.jpg" alt="">
        <img src="./bojie.jpg" alt="">
        <img src="./bojie.jpg" alt="">
        <img src="./bojie.jpg" alt="">
        <img src="./bojie.jpg" alt="">
        <img src="./bojie.jpg" alt="">
        <img src="./bojie.jpg" alt="">

    </div>

    <script src="./js/jquery-3.2.1.min.js"></script>
    <script type='text/javascript'>
        
        // 这下面的代码后面咱们会讲，大家不用在这个初学阶段去纠结下面的代码。
        $(function(){
            $('p').click(function(){
            
                $('html').animate({
                    "scrollTop":0
                },2000)
            })
        })
    </script>
</body>
</html>

5、z-index

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style type="text/css">
        
        *{
            padding: 0;
            margin: 0

        }
        .box1{
            width: 200px;
            height: 200px;
            background-color: red;
            position:relative;
            top: 30px;
            left: 40px;
            z-index: 3;
        }
        .box2{
            width: 200px;
            height: 200px;
            background-color: yellow;
            position: relative;
            top: 0;
            left: 0;
            z-index: 2;
        }
        .box3{
            width: 200px;
            height: 200px;
            background-color: green;
            float: left;
            margin-left: 20px;
            margin-top: -30px;

        }
    </style>
</head>
<body>


      <div class="box1"></div>
     <div class="box2"></div>
     <div class="box3"></div>


    
</body>
</html>

　　（1）从父现象

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style type="text/css">
        *{
            margin: 0;
            padding: 0
        }
        .tianliang{
            width: 200px;
            height: 200px;
            background-color: red;
            position: relative;
            z-index: 3;
        
        }
        .tianliang .sendie{
            width: 100px;
            height: 100px;
            background-color: pink;
            position: absolute;
            top: 240px;
            left: 300px;
            z-index: 333;
            
        }
        .lzy{
            width: 200px;
            height: 200px;
            background-color: yellow;
            position: relative;
            z-index: 4;
        }
        .lzy .brother{
            width: 100px;
            height: 100px;
            background-color: green;
            position: absolute;
            top: 100px;
            left: 320px;
            z-index: 111;
        
        }
    </style>
</head>
<body>
    
    <div class="tianliang">
        <p class="sendie"></p>
    </div>
    <div class="lzy">
        <p class="brother"></p>
    </div>
</body>
</html>

　　（2）固定导航栏加9999

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原文地址：https://www.cnblogs.com/venicid/p/9126919.html