Detailed explanation of Float attribute in CSS-CSS Tutorial-php.cn

This time I will bring you a detailed explanation of the Float attribute in CSS. What are the precautions for using the Float attribute in CSS? . Here is a practical case, let's take a look.

1. Characteristics of Float

1. Apply text around images

2. Create a block-level box

3. Multiple columnsFloating layout

4. The width and height of floating elements are adaptive, but their values can be set.

2. Core solved problems

Text surrounds the image: img tagPlace multiple text labels in one In the container, if the img is floated, the text label will surround the image.

<img src="../img/a.jpg" style=" width: 100px; height: 200px; float: left;" alt="">
        <p>001文件内容文件内容文内容文件内容<br/>
        文件内容文件内容文件内容文件内容文件内内容文件内容<br/>
        文件内容文件内容文件内容文件内容文件内容<br/> 
        文件内容文件内容文件内容文件内容文内容文件内容文件内容<br/>
        <p>p标签文件内容文件内容文件内容文件内容文件内容文容文件内容文件内容文件内容</p>
        <p>p标签文件内容文件内容文件内容文件内容文件内容文件内容</p>
        文件内容文件内容文件内容文件件内容文件内容文件内容<br/>
        文件内容文件内容文件内容文件内容文件内容文件内容文件内容文件内容<br/> 
        文件内容文件内容文件内容文件内容文件内容文件内内容文件内容<br/>
        文件内容文件内容文件内容文件容文件内容文件内容<br/>
        文件内容文件内容文件内容文件容文件内容文件内容<br/>
        </p>

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2.1 This is a problem

The floating element is adjacent to the normal element, and there is no gap between the floating element and the normal element Clear float. At this time, the normal element will be covered by the floating element, but the contained content will be displayed around the floating element.

<p style="width: 100px; height: 200px; background: red;float: left;" >001</p> 
<p style="width: 100px; height: 200px; background: gray;float: none;" ><p>002</p></p>

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001 floats, 002 does not float, but the 002 element itself is covered by 001, but the content is displayed around 001.

3. Non-core and main application areas

Column layout: let the blocks be horizontal first Arrange, and then start a new line with the excess part.

Main features

1. Parent height collapse (this is also a serious problem)

.wrap{
           background:red; 
           padding:10px;
           width:auto;
        }
        .left{
            background:gray;
            width:200px;
            height:100px;
            float:left;
        }
        .right{
            background:yellow;
            width:100px;
            height:100px;
            float:left;
        }

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<p class="wrap">
        <p class="left">left</p>
        <p class="right">right</p>
</p>

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2. Width and height changes into an adaptive sub-element, but the width and height settings are valid

.wrap{
           background:red; 
           padding:10px;
           float:left;
        } 
        .left{
            width:100px;
            background:gray;
        }
        .right:{
            width:200px;
            background:yellow;
        }

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<p class="wrap">
        <p class="left">left</p>
        <p class="right">right</p>
</p>

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2. Solve the problem of height collapse

First we You need to understand the two basic concepts of BFC and IFC, because they are closely related to browser rendering.

1.BFC (block-level formatting context)

It is an independently rendered area, stipulating how to lay out the interior of the area, and has nothing to do with the outside. The main rules are as follows:

1.1 The internal boxes will be placed vertically, one after another

1.2 The vertical distance of the Box is determined by margin, and the margins of two adjacent boxes belonging to the same BFC will overlap

1.3 The BFC area will not overlap with float

.head{
            background:pink;
            margin: 20px 0px;
            height:100px;
        }
        .wrap{
           background:red; 
           padding:10px;
           margin:20px 0px;
           overflow:hidden;
        } 
        .left{
            width:100px;
            background:gray; 
            margin:10px 0px;
        }
        .right:{
            width:200px;
            background:yellow;  
            margin:20px 0px;
        }

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<p class="head">head</p>
    <p class="wrap">
        <p class="left">left</p>
        <p class="right">right</p>
    </p>

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There are 20px margins between the two boxes of .head and .wrap, but they overlap. ;

Between .head and .left, .head has a 20px margin and .left has a 10px margin. There is no overlap because .wrap creates a BFC (overflow:hidden) .

1.4 The left margin of each box is in contact with the left side of the containing border box (the same is true for the right side), and the same is true for floats

2. IFC (row-level formatting context)

Boxes are placed horizontally one after the other starting from the top of the containing block. The space occupied by the horizontal margins, borders, and padding are all put together (display is inline, inline-block; elements with inline characteristics have the following characteristics). The rules are as follows:

2.1 The width and height cannot be specified

2.2 Margin, Padding, and border are invalid in the vertical direction

2.3 The left side of the line box is close to the left side of the containing block, and the line The right side of a box is flush with the right side of its containing box, and a float can be inserted between the edge of the containing block and the line box.

2.4 The height of the inline box is determined by line-height.

For examples in this section, please refer to the inline element in the display chapter.

3. Solution

主要根据BFC的原理实现，因为BFC的渲染的是整块区域，也就会计算出宽、高。这也是传说中的清除浮动的方案

3.1 父容器创建BFC方法

3.1.1 创建BFC的方法

a) Float除了none以外的取值；

b) Overflow除了visible以外的值；

c) Display值为table-cell、table-caption、inline-block、flex、inline-flex等

d) Position值为absloute、fixed

e) Fieldset元素

3.1.2 清除浮动

a) Float、overflow、display三种方式都可以清除浮动，但position、fieldset虽然创建了bfc但不可以清除浮动（也就是不能解决高度塌陷的问题）。主要原因为：position、fieldset都需要子元素来撑开父容器的高度，但子元素浮动后又不存在高度，所以失效。

b) Float、overflow、display示例代码:

.wrap{
            background: gray;
            padding: 10px;
            overflow: auto;
        }
        .left, .right{
            background: red;
            float: left;
            width: 200px;
            height: 100px;
        }
        .right{
            background: yellow;
        }
        .footer{
            background: pink;
        }

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<p class="wrap" >
        <p class="left">left</p>
        <p class="right">right</p>
    </p>
<p class="footer">footer</p>

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3.1.3 最后一个子元素clear:both

利用clear:both触发父容器重新计算高度的原理实现，示例代码如下：

.wrap{
            background: gray;
            padding: 10px; 
        }
        .left, .right{
            background: red;
            float: left;
            width: 200px;
            height: 100px;
        }
        .right{
            background: yellow;
        }
        .footer{
            background: pink;
        }
        .clear{
            clear: both;
            zoom: 1;
        }

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<p class="wrap" >
        <p class="left">left</p>
        <p class="right">right</p>
        <p class="clear"></p>
    </p>
<p class="footer">footer</p>

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3.1.4 After添加最后一个子元素

利用css的:after伪元素实现，动态插入元素并清除浮动：

.wrap{
            background: gray;
            padding: 10px; 
        }
        .wrap:after{
            content: '';
            display: block;
            overflow: hidden;
            clear: both;
        }
        .left, .right{
            background: red;
            float: left;
            width: 200px;
            height: 100px;
        }
        .right{
            background: yellow;
        }
        .footer{
            background: pink;
        }

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<p class="wrap" >
        <p class="left">left</p>
        <p class="right">right</p> 
    </p>
<p class="footer">footer</p>

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4. 总结

1. 利用bfc方式清除浮动，简单、浏览器支持良好，但在IE6-版本支持存在问题。但是存在以下局限性，要适环境而用：

a) Overflow方式：滚动条会被隐藏，如果子内容超高则存在显示不全的问题；

b) Float方式：让父容器浮动，那么就存在对父容器同辈元素的影响；

c) Dipslay方式：让父容器变为table或者flex等，都存在不明确的影响，大家都不推荐使用。

2. 最佳解决方案：利用:after添加一个伪元素并给予clear:both和zoom:1来实现清除浮动，兼容性好，对环境影响最小。

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