Analysis of CSS float and position-CSS Tutorial-php.cn

This article focuses on learning and understanding CSS floating float and positioning, and uses examples to help you master CSS floating float and positioning skills. Interested friends can refer to

1. Floating float

I. Definition and rules

float defaults to none, corresponding to the standard stream. When float: left;, the element will move closer to the left side of its parent element and break away from the standard flow. At the same time, the width will no longer stretch to fill the parent container, but will be determined based on its own content.

II. Demonstration rules

Preparation code

<html xmlns="http://www.w3.org/1999/xhtml">  
<head>  
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />  
    <title></title>  
    <style>  
        body   
        {   
            margin: 0;   
            padding: 0;   
        }   
  
        #father   
        {   
            background-color: cyan;   
  
            /*父级p 没有定位 造成子p的margin-top传递给父级*/   
            position: absolute;   
        }   
  
            #father *   
            {   
                margin: 10px;   
                padding: 10px;   
                border: 1px dashed red;   
            }   
  
        #son1   
        {   
        }   
  
        #son2   
        {   
        }   
  
        #son3   
        {   
        }   
    </style>  
</head>  
<body>  
    <p id="father">  
        <p id="son1">#son1</p>  
        <p id="son2">#son2</p>  
        <p id="son3">#son3-son3son3son3</p>  
        <p>  
        这是文字这是文字这是文字这是文字这是文字这是文字这是文字这是文字这是文字这是文字这是文字这是文字这是文字这是文字这是文字这是文字这是文字这是文字这是文字这是文字   
        </p>  
    </p>  
</body>  
</html>

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1. Add position:absolute to #father in the middle to eliminate the margin-top transfer problem of unpositioned parent p , the relevant content is as follows

Solution to the margin-top transfer problem in nested p

In these two browsers , there are two nested ps. If the padding value of the parent element of the outer p is 0, then the margin-top or margin-bottom value of the inner p will be "transferred" to the outer p.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">  
<html xmlns="http://www.w3.org/1999/xhtml">  
<head>  
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />  
<title>无标题文档</title>  
</head>  
  
<body>  
<p style="background-color:#FF0000;width:300px; height:100px">上部层</p>  
  
<p style="background-color:#009900; width:300px; height:300px;overflow:hidden "> <!--父层-->  
     <p style="margin:50px; background-color:#000000;width:200px; height:200px"">子层</p>  
</p>  
  
</body>  
</html>

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Cause: The box does not obtain haslayout, causing margin-top to be invalid

Solution:
1. Add: overflow:hidden to the parent layer p;
2. Put margin -top outer margin is changed to padding-top inner margin;
3. The side where the parent element's margin overlaps has padding that is not 0 or a border whose width is not 0 and style is not none.
Add p to the parent layer: padding-top: 1px;
4. Let the parent element generate a block formatting context. The following attributes can be implemented
* float: left/right
* position: absolute
* display: inline-block/table-cell (or other table types)
* overflow: hidden/auto
Add parent layer p: position: absolute;

The display effect is

When the floats of 2, 1, and 2 are left and right respectively,

can be seen that 1 and 2 are out of standard stream, son3 in the standard stream is treated as if they do not exist, so son3 replaces the original position of son1, and the left border of son1, the right border of son2 coincide with the left and right borders of son3

3. When 1,2,3 When all float left

, the text surrounds the floated p

4, 1, 2 float left, 3 floats right, when the window width is reduced, 3 will be squeezed down

When 3 floats to the left and 2 floats to the right, it will be displayed as

when browsing When the width of the browser window is reduced, guess who will be squeezed out, son2?

The answer is still son3. The rule is: The last words written in the html file will be squeezed out. In the html file, son3 is after son2, so son3 is always squeezed first. Come down.

5. Increase the height of son1. When son3 is squeezed down, it will get stuck there

6. Delete the text in the box and float all three sub-p's to the left
Displayed as

The three child ps in the parent p are all out of the standard stream, and the parent p shrinks into a line. You can use clear to correct it
Add an empty p with margin-padding-border all 0 and clear both to support the parent p

III. clear clears the float
If the previous Elements with float:left will affect the following elements, such as p in the above example. Writing clear: left in the p element can eliminate the impact of the previous left floating element on this element. In the same way, clear:both clears both the left and right elements.

2. Position position

The position value is static absolute relative fixed

1. static
This is the default, that is, standard flow arrangement Next, it is the static positioning method.

2. fixed
It is fixed in the browser window. The [Back to Top] button in the forum is made of fixed
Practice making a return Play at the top

<p id="backToTop">   
    回到顶部   
</p>   
#backToTop   
{   
    width: 100px;   
    height: 50px;   
  
  
    background-color: red;   
    color: white;   
    cursor: pointer;   
    border-radius: 25px 0 0 25px;   
    padding-left: 20px;   
  
  
    text-align: center;   
    line-height: 50px;   
  
    position: fixed;   
    bottombottom: 80px;   
    rightright: 0;   
}

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Display effect

3. Relative relative positioning

Relative to its own offset, and not deviating from the standard flow, use top/bottom left/right to specify the offset

4. Absolute positioning

Position based on other positioned elements, using absolute rules to break away from the standard flow

1), this other element:
The closest positioned ancestor element or browser Window, when the previous ancestor element cannot be found, the latter browser window will be used to position it.
2), Already positioned: means that the position has been set, and it is not static...that is, the position value is either static or already. If a positioned element is not set to position or is set to static, it is considered not to be positioned.
Trick

Only set position: absolute without setting the top/bottom/left/right value, then the element will remain in place , but it has been separated from the standard flow.

3. display

The display value has inline block none

Set it to none to hide it , such as inline-block and other newly added

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