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Detailed explanation of the usage of Position in CSS.

WBOY
Release： 2016-08-10 08:49:41
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I remember a year ago, when I went to a company for an interview, I was asked what attributes position had. It took me a long time to answer 2. Everyone probably knows that they are the 2 we often use (relative, absolute).

I have used it a lot recently and studied it in depth.

1. Grammar:

position: static | relative | absolute | fixed

Value:
static: Default value, no special positioning, the object follows normal document flow. Attributes such as top, right, bottom, and left will not be applied and have default values.
relative: Relative positioning, the object follows the normal document flow, but will be offset in the normal document flow based on top, right, bottom, left and other attributes.
Absolute: Absolute positioning, the object is separated from the normal document flow, and attributes such as top, right, bottom, and left are used for absolute positioning. And its cascading is defined through the z-index attribute.
fixed: Fixed positioning. The object is separated from the normal document flow. Use top, right, bottom, left and other attributes to position the window as a reference point. When the scroll bar appears, the object will not scroll with it.

Detailed introduction of the second and four attributes:

1, static, default value
At this point we have to mention a definition - document flow. Many books only briefly mention the concept of document flow without giving a detailed answer. However, I summarized it based on my own experience and understanding. Document flow is actually is the output order of the document, which is the output form we usually see from left to right, top to bottom. Each element in the web page is sorted and displayed in this order, and float and The two position attributes can separate elements from the document flow and display them.

The default value is to let the element continue to display according to the document flow without making any changes.

2, relative, relative positioning

One of the biggest features of relative positioning is that it still occupies the original position after running away through positioning, and will not give it to surrounding objects such as text flow. Relative positioning is also relatively independent. It has the final say on what to do. When it is positioned, it is offset by its own position (relative to the object itself). The position itself has changed, but the physical space originally occupied by the element still exists. In addition, the relative positioning of another element does not affect other adjacent elements.

3, absolute, absolute positioning

After the position is set to absolute, the set element will be defined to the specified coordinates (X coordinate, Y coordinate) within the element containing it.

For example: position: absolute; left: 20px; top: 80px; This container is always located 20px from the left of the browser and 80px from the top of the browser. It will not change with the window size, it is just fixed on a specific coordinate axis;

4, fixed position, fixed

In theory, an element set to fixed will be positioned at a specified coordinate in the browser window, and it will be fixed at this position regardless of whether the window is scrolled or not. But currently it is not supported in IE6.

Ie6 processing method: 

<span style="color: #800000;">body</span>{<span style="color: #ff0000;">
    background-image</span>:<span style="color: #0000ff;">url(about:blank)</span>; <span style="color: #008000;">/*</span><span style="color: #008000;"> for IE6 </span><span style="color: #008000;">*/</span><span style="color: #ff0000;">
    background-attachment</span>:<span style="color: #0000ff;">fixed</span>; <span style="color: #008000;">/*</span><span style="color: #008000;">必须</span><span style="color: #008000;">*/</span>
}<span style="color: #800000;">

.demo</span>{<span style="color: #ff0000;">
    _position</span>:<span style="color: #0000ff;">absolute</span>;<span style="color: #ff0000;">
    _top</span>:<span style="color: #0000ff;">expression(documentElement.scrollTop + documentElement.clientHeight-this.offsetHeight)</span>;
}
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Total example html: 

<span style="color: #0000ff;"><!</span><span style="color: #ff00ff;">DOCTYPE html</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">html</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">head</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">meta </span><span style="color: #ff0000;">charset</span><span style="color: #0000ff;">="UTF-8"</span> <span style="color: #0000ff;">/></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">title</span><span style="color: #0000ff;">></span>position<span style="color: #0000ff;"></</span><span style="color: #800000;">title</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">style </span><span style="color: #ff0000;">type</span><span style="color: #0000ff;">="text/css"</span><span style="color: #0000ff;">></span><span style="background-color: #f5f5f5; color: #800000;">
*</span><span style="background-color: #f5f5f5; color: #000000;">{</span><span style="background-color: #f5f5f5; color: #ff0000;">margin</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">0</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">padding</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">0</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #000000;">}</span><span style="background-color: #f5f5f5; color: #800000;">
.clearfix:before,.clearfix:after </span><span style="background-color: #f5f5f5; color: #000000;">{</span><span style="background-color: #f5f5f5; color: #ff0000;">content</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">""</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">display</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">table</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #000000;">}</span><span style="background-color: #f5f5f5; color: #800000;">
.clearfix:after </span><span style="background-color: #f5f5f5; color: #000000;">{</span><span style="background-color: #f5f5f5; color: #ff0000;">clear</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">both</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">overflow</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">hidden</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #000000;">}</span><span style="background-color: #f5f5f5; color: #800000;">
.clearfix </span><span style="background-color: #f5f5f5; color: #000000;">{</span><span style="background-color: #f5f5f5; color: #ff0000;">zoom</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">1</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #000000;">}</span><span style="background-color: #f5f5f5; color: #800000;">
.mgt20</span><span style="background-color: #f5f5f5; color: #000000;">{</span><span style="background-color: #f5f5f5; color: #ff0000;">margin-top</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">20px</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #000000;">}</span><span style="background-color: #f5f5f5; color: #800000;">
.w1000</span><span style="background-color: #f5f5f5; color: #000000;">{</span><span style="background-color: #f5f5f5; color: #ff0000;">width</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">1000px</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">margin</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">0 auto</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">min-height</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">1900px</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #000000;">}</span><span style="background-color: #f5f5f5; color: #800000;">

#demo</span><span style="background-color: #f5f5f5; color: #000000;">{</span><span style="background-color: #f5f5f5; color: #ff0000;">  
    position</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">relative</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">  
    border</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">1px solid #000</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">  
    margin-top</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">50px</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">
    top</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">-50px</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">  
    line-height</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">18px</span><span style="background-color: #f5f5f5; color: #000000;">;</span> 
<span style="background-color: #f5f5f5; color: #000000;">}</span><span style="background-color: #f5f5f5; color: #800000;"> 
.static,.fixed,.absolute,.relative</span><span style="background-color: #f5f5f5; color: #000000;">{</span><span style="background-color: #f5f5f5; color: #ff0000;">  
    width</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">200px</span><span style="background-color: #f5f5f5; color: #000000;">;</span>      
<span style="background-color: #f5f5f5; color: #000000;">}</span><span style="background-color: #f5f5f5; color: #800000;">  
.relative</span><span style="background-color: #f5f5f5; color: #000000;">{</span><span style="background-color: #f5f5f5; color: #ff0000;">  
    position</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">relative</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">  
    left</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">150px</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">  
    top</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">-20px</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">  
    background-color</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">#ffe4e1</span><span style="background-color: #f5f5f5; color: #000000;">;</span>  
<span style="background-color: #f5f5f5; color: #000000;">}</span><span style="background-color: #f5f5f5; color: #800000;">
.static</span><span style="background-color: #f5f5f5; color: #000000;">{</span><span style="background-color: #f5f5f5; color: #ff0000;">   
    position</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">static</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;"> 
    background-color</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">#bbb</span><span style="background-color: #f5f5f5; color: #000000;">;</span>    
<span style="background-color: #f5f5f5; color: #000000;">}</span><span style="background-color: #f5f5f5; color: #800000;">  
.fixed</span><span style="background-color: #f5f5f5; color: #000000;">{</span><span style="background-color: #f5f5f5; color: #ff0000;">  
    position</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">fixed</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">  
    top</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">200px</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">
    left</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">50%</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">
    margin-left</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">-500px</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">
    background-color</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">#ffc0cb</span><span style="background-color: #f5f5f5; color: #000000;">;</span>
<span style="background-color: #f5f5f5; color: #000000;">}</span><span style="background-color: #f5f5f5; color: #800000;">  
.absolute</span><span style="background-color: #f5f5f5; color: #000000;">{</span><span style="background-color: #f5f5f5; color: #ff0000;"> 
    position</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">absolute</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">  
    right</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">10px</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">  
    top</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">10px</span><span style="background-color: #f5f5f5; color: #000000;">;</span><span style="background-color: #f5f5f5; color: #ff0000;">  
    background-color</span><span style="background-color: #f5f5f5; color: #000000;">:</span><span style="background-color: #f5f5f5; color: #0000ff;">#b0c4de</span><span style="background-color: #f5f5f5; color: #000000;">;</span>  
<span style="background-color: #f5f5f5; color: #000000;">}</span> 
<span style="color: #0000ff;"></</span><span style="color: #800000;">style</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"></</span><span style="color: #800000;">head</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">body</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">div </span><span style="color: #ff0000;">class</span><span style="color: #0000ff;">="w1000"</span><span style="color: #0000ff;">></span>
    <span style="color: #0000ff;"><</span><span style="color: #800000;">div </span><span style="color: #ff0000;">class</span><span style="color: #0000ff;">="mgt20 clearfix"</span><span style="color: #0000ff;">></span>
        <span style="color: #0000ff;"><</span><span style="color: #800000;">div </span><span style="color: #ff0000;">id</span><span style="color: #0000ff;">="demo"</span><span style="color: #ff0000;"> class</span><span style="color: #0000ff;">="clearfix"</span><span style="color: #0000ff;">></span>  
            <span style="color: #0000ff;"><</span><span style="color: #800000;">div </span><span style="color: #ff0000;">class</span><span style="color: #0000ff;">="static"</span><span style="color: #0000ff;">></span>static: 默认值。无特殊定位，对象遵循HTML定位规则 <span style="color: #0000ff;"></</span><span style="color: #800000;">div</span><span style="color: #0000ff;">></span>  
            <span style="color: #0000ff;"><</span><span style="color: #800000;">div </span><span style="color: #ff0000;">class</span><span style="color: #0000ff;">="absolute"</span><span style="color: #0000ff;">></span>absolute: 将对象从文档流中拖出,使用left,right,top,bottom 等属性相对于其最接近的一个最有定位设置的父对象进行绝对定位。如果不存在这样的父对象,则依据body对象。而其层叠通过z-index属性定义 <span style="color: #0000ff;"></</span><span style="color: #800000;">div</span><span style="color: #0000ff;">></span>  
            <span style="color: #0000ff;"><</span><span style="color: #800000;">div </span><span style="color: #ff0000;">class</span><span style="color: #0000ff;">="fixed"</span><span style="color: #0000ff;">></span>fixed属性的说明：fixed总是以body为定位时的对象，总是根据浏览器的窗口来进行元素的定位，通过"left"、 "top"、 "right"、"bottom" 属性进行定位。<span style="color: #0000ff;"></</span><span style="color: #800000;">div</span><span style="color: #0000ff;">></span>  
            <span style="color: #0000ff;"><</span><span style="color: #800000;">div </span><span style="color: #ff0000;">class</span><span style="color: #0000ff;">="relative"</span><span style="color: #0000ff;">></span>relative1:对象不可层叠，对象原来占有的位置保留,但将依据 left,right,top,bottom 等属性在正常文档流中偏移位置 <span style="color: #0000ff;"></</span><span style="color: #800000;">div</span><span style="color: #0000ff;">></span> 
        <span style="color: #0000ff;"></</span><span style="color: #800000;">div</span><span style="color: #0000ff;">></span> 
    <span style="color: #0000ff;"></</span><span style="color: #800000;">div</span><span style="color: #0000ff;">></span>    
<span style="color: #0000ff;"></</span><span style="color: #800000;">div</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"></</span><span style="color: #800000;">body</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"></</span><span style="color: #800000;">html</span><span style="color: #0000ff;">></span>
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显示：

 

三、position注意事项

1、当Position属性值为Relative时对象原来占有的位置保留，其后面 的对象按原来文档流仍然保持原来的位置Top的值表示对象相对原位置向下偏移的距离bottom的值表示对象相对原位置向上偏移的距离两者同时存在时，只有Top起作用。left的值表示对象相对原位置向右偏移的距离right的值表示对象相对原位置向左偏移的距离两者同时存在时，只有left起作用。

2、当Position属性值为absolute时对象从文档流中抽取出来，原占有的位置被后面的对象顶替上来Top的值表示对象上边框与浏览器窗口顶部 的距离bottom的值表示对象下边框与浏览器窗口底部的距离两者同时存在时，只有Top起作用；如果两者都未指定，则其顶端将与原文档流位置一致，即垂 直保持位置不变。left的值表示对象左边框与浏览器窗口左边的距离right的值表示对象右边框与浏览器窗口右边的距离两者同时存在时，只有left起 作用；如果两者都未指定，则其左边将与原文档流位置一致，即水平保持位置不变。
在Position属性值为absolute的同时，如果有一级父对象（无论是父对象还是祖父对象，或者再高的辈分，一样）的Position属性值为Relative时，则上述的相对浏览器窗口定位将会变成相对父对象定位，这对精确定位是很有帮助的。

 

五、positon 与 display
元素分为内联元素和区块元素两类（当然也有其它的），在内联元素中有个非常重要的常识，即内联元素是不可以设置区块元素所具有的样式，例如:width | height。
relative : 原来是什么类型的依旧是什么类型。
absolute | fixed ： 元素就被重置为了区块元素，

例如：打算为span元素指定大小，并绝对定位

<span style="color: #0000ff;"><</span><span style="color: #800000;">span </span><span style="color: #ff0000;">style</span><span style="color: #0000ff;">="position:absolute; width:100px; height:50px;"</span><span style="color: #0000ff;">></span>span<span style="color: #0000ff;"></</span><span style="color: #800000;">span</span><span style="color: #0000ff;">></span><span style="color: #000000;">这是完全正确的，
</span><span style="color: #0000ff;"><</span><span style="color: #800000;">span </span><span style="color: #ff0000;">style</span><span style="color: #0000ff;">="position:absolute; display:block; width:100px; height:100px;"</span><span style="color: #0000ff;">></span>span<span style="color: #0000ff;"></</span><span style="color: #800000;">span</span><span style="color: #0000ff;">></span>，这里的display:block就是多余的了。
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六、position 与 float
一个元素若设置了 position:absolute | fixed; 则该元素就不能设置float。这是一个常识性的知识点，因为这是两个不同的流，一个是浮动流，另一个是“定位流”。
但是 relative 却可以。因为它原本所占的空间仍然占据文档流。

Related labels：
CSS中Position 的用法详解。
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