CSS规则清楚：两者都是？ [英] What does the CSS rule clear: both do?

问题描述

以下CSS规则执行的操作：

  .clear {clear：both; } 
  

为什么我们需要使用它？

解决方案

我不会解释浮动在这里的工作（详细），因为这个问题通常集中在为什么使用 ; OR clear：both; 完全正确...

我会保持这个答案很简单，到了点，并会解释为你 clear：both; 是必需的或它做什么...

一般来说，设计师将元素向左或向右浮动，这会在另一侧创建一个空白空间，允许其他元素占据剩余空间。

为什么要浮动元素？

当设计者需要2个块级元素时，元素是浮动的。例如，我们想设计一个基本网站，其布局如下：

所述，我们清除这些浮动广告的方式不是干净的方式，因为我们使用一个空的 div 元素不是 div 元素。

想象它是一个虚拟元素，它会在你的父元素结束之前为你创建一个空元素。这将自动清除包含浮动元素的包装元素。

要自我清除任何包含浮动元素的包装器元素，我们可以使用

  .wrapper_having_floated_elements：after {/ * Imaginary class name * / 
 content：; 
 clear：both; 
 display：table; 
} 
  

注意：我使用的伪元素 class 。这将在包装器元素关闭之前创建一个虚拟元素。如果我们查看dom，您可以看到它在文档树中是如何显示的。

所以，如果你看到，它是浮动的子 div 后，我们清除浮动，这是什么，只是相当于有一个空 div 元素与 clear：both; 属性，我们也为此使用。现在为什么 display：table; 和 content 超出了这个答案范围， https://developer.mozilla.org/en-US/docs/Web/CSS/Pseudo-elements =nofollow noreferrer>此处为伪元素。

请注意，这也将在IE8中作为 IE8支持：after pseudo 。

---

原始答案：

大多数开发人员将他们的内容左右悬浮在他们的页面上，可能是div包含徽标，侧边栏，内容等，这些div是左或右浮动，剩下的空间未使用，因此，如果你放置其他容器，它将在剩余空间中浮动，因此为了防止 clear：both; ，它会清除所有向左或向右浮动的元素。 p>

演示：

  ------- ---------------------------------- 
 div1（左移）其他div占用了这里的空间
 ------------------ --------------------- ------------- 
  

现在如果你想其他div会显示在 div1 下，所以你会使用 clear：both; 左或右

  ------------------ 
 div1 （左浮动）
 ------------------ 
< div style =clear：both;><！ -  This< ; div>充当分隔符 - >< / div> 
 ---------------------------------- 
其他div现在显示为
 ---------------------------------- 
  

What does the following CSS rule do:

.clear { clear: both; }

And why do we need to use it?

解决方案

I won't be explaining how the floats work here (in detail), as this question generally focuses on Why use clear: both; OR what does clear: both; exactly do...

I'll keep this answer simple, and to the point, and will explain to you graphically why clear: both; is required or what it does...

Generally designers float the elements, left or to the right, which creates an empty space on the other side which allows other elements to take up the remaining space.

Why do they float elements?

Elements are floated when the designer needs 2 block level elements side by side. For example say we want to design a basic website which has a layout like below...

Live Example of the demo image.

Code For Demo

/*  CSS:  */

* { /* Not related to floats / clear both, used it for demo purpose only */
    box-sizing: border-box;
    -moz-box-sizing: border-box;
    -webkit-box-sizing: border-box;
}

header, footer {
    border: 5px solid #000;
    height: 100px;
}

aside {
    float: left;
    width: 30%;
    border: 5px solid #000;
    height: 300px;
}

section {
    float: left;
    width: 70%;
    border: 5px solid #000;
    height: 300px;
}

.clear {
    clear: both;
}

<!-- HTML -->
<header>
    Header
</header>
<aside>
    Aside (Floated Left)
</aside>
<section>
    Content (Floated Left, Can Be Floated To Right As Well)
</section>
<!-- Clearing Floating Elements-->
<div class="clear"></div>
<footer>
    Footer
</footer>

Note: You might have to add header, footer, aside, section (and other HTML5 elements) as display: block; in your stylesheet for explicitly mentioning that the elements are block level elements.

Explanation:

I have a basic layout, 1 header, 1 side bar, 1 content area and 1 footer.

No floats for header, next comes the aside tag which I'll be using for my website sidebar, so I'll be floating the element to left.

Note: By default, block level element takes up document 100% width, but when floated left or right, it will resize according to the content it holds.

1. Normal Behavior Of Block Level Element
2. Floated Behavior Of Block Level Element

So as you note, the left floated div leaves the space to its right unused, which will allow the div after it to shift in the remaining space.

1. div's will render one after the other if they are NOT floated
2. div will shift beside each other if floated left or right

Ok, so this is how block level elements behave when floated left or right, so now why is clear: both; required and why?

So if you note in the layout demo - in case you forgot, here it is..

I am using a class called .clear and it holds a property called clear with a value of both. So lets see why it needs both.

I've floated aside and section elements to the left, so assume a scenario, where we have a pool, where header is solid land, aside and section are floating in the pool and footer is solid land again, something like this..

So the blue water has no idea what the area of the floated elements are, they can be bigger than the pool or smaller, so here comes a common issue which troubles 90% of CSS beginners: why the background of a container element is not stretched when it holds floated elements. It's because the container element is a POOL here and the POOL has no idea how many objects are floating, or what the length or breadth of the floated elements are, so it simply won't stretch.

1. Normal Flow Of The Document
2. Sections Floated To Left
3. Cleared Floated Elements To Stretch Background Color Of The Container

(Refer ^[Clearfix] section of this answer for neat way to do this. I am using an empty div example intentionally for explanation purpose)

I've provided 3 examples above, 1st is the normal document flow where red background will just render as expected since the container doesn't hold any floated objects.

In the second example, when the object is floated to left, the container element (POOL) won't know the dimensions of the floated elements and hence it won't stretch to the floated elements height.

After using clear: both;, the container element will be stretched to its floated element dimensions.

Another reason the clear: both; is used is to prevent the element to shift up in the remaining space.

Say you want 2 elements side by side and another element below them... So you will float 2 elements to left and you want the other below them.

1. div Floated left resulting in section moving into remaining space
2. Floated div cleared so that the section tag will render below the floated divs

---

1st Example

---

2nd Example

Last but not the least, the footer tag will be rendered after floated elements as I've used the clear class before declaring my footer tags, which ensures that all the floated elements (left/right) are cleared up to that point.

---

Clearfix

Coming to clearfix which is related to floats. As already specified by @Elky, the way we are clearing these floats is not a clean way to do it as we are using an empty div element which is not a div element is meant for. Hence here comes the clearfix.

Think of it as a virtual element which will create an empty element for you before your parent element ends. This will self clear your wrapper element holding floated elements. This element won't exist in your DOM literally but will do the job.

To self clear any wrapper element having floated elements, we can use

.wrapper_having_floated_elements:after {  /* Imaginary class name */
  content: "";
  clear: both;
  display: table;
}

Note the :after pseudo element used by me for that class. That will create a virtual element for the wrapper element just before it closes itself. If we look in the dom you can see how it shows up in the Document tree.

So if you see, it is rendered after the floated child div where we clear the floats which is nothing but equivalent to have an empty div element with clear: both; property which we are using for this too. Now why display: table; and content is out of this answers scope but you can learn more about pseudo element here.

Note that this will also work in IE8 as IE8 supports :after pseudo.

---

Original Answer:

Most of the developers float their content left or right on their pages, probably divs holding logo, sidebar, content etc., these divs are floated left or right, leaving the rest of the space unused and hence if you place other containers, it will float too in the remaining space, so in order to prevent that clear: both; is used, it clears all the elements floated left or right.

Demonstration:

------------------ ----------------------------------
div1(Floated Left) Other div takes up the space here
------------------ ----------------------------------

Now what if you want to make the other div render below div1, so you'll use clear: both; so it will ensure you clear all floats, left or right

------------------
div1(Floated Left)
------------------
<div style="clear: both;"><!--This <div> acts as a separator--></div>
----------------------------------
Other div renders here now
----------------------------------

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