search
HomeWeb Front-endHTML TutorialCSS2系列:外边距合并问题(margincollapse)_html/css_WEB-ITnose

外边距合并

当两个垂直方向外边距相遇,它们将形成一个折叠外边距。

合并后的外边距的高度等于两个发生合并的外边距的高度中的较大者。
为了好认识,我们称内部div为"盒子",包含这个内部div的外部div为"包含体"

解释

  • 怎么相遇:
  • 相邻的两个盒子(可能是兄弟关系也可能是包含关系)这两个元素margin之间不能夹杂着边框啊,补白啊什么的就算margin相遇,这点很重要
  • 哪个方向
  • 一定为垂直方向,水平方向没有
  • 形成一个折叠外边距过程
    1. 盒子margintop和包含体margintop取出最终折叠外边距值。下面为取这个值的情况 150 50=150 150 -50=100 -150 -50 =-150 -150 50 =-100
    2. 两个相邻的外边距都是正数时,折叠结果是它们两者之间较大的值。可以在下面例子2中,改变一下
    3. 两个相邻的外边距都是负数时,折叠结果是两者绝对值的较大值。
    4. 两个外边距一正一负时,折叠结果是两者的相加的和。
    5. 包含关系时,盒子不显示margin-top(相当没有margin-top),包含体显示margin-top这个最终值。
    6. 兄弟关系时,两兄弟之间的margin共用这个最终值。
  • 极限情况一个元素

  • 当一个块元素,没有补白和填充,上外边距与下外边距就碰到了一起,它们也会发生合并。
  • 例子1包含关系,不设置外层div的内边距(补白)和边框的前提!

    <meta charset="utf-8"><style type="text/css">            * {margin:0;padding:0;border:0;}            #outer {width:200px;height:200px;background-color:red;margin-top:-20px;}            #inner {width:50px;height:50px;background-color:blue;margin-top:50px;}</style>

    <div id="outer">  <div id="inner">  </div></div><!--不设置 div 的内边距和边框,那么内部 div 的上外边距将与外部 div 的上外边距合并(叠加),内部div箱单更木有margin-top。外部用这个最大值。-->

    例子2兄弟关系

    * {margin:0; padding:0;border:0;}#d1 {width:100px;height:100px;margin-top:50px;margin-bottom:50px;background-color:red;}/*改变margin-bottom:-50px;试一试*/#d2 {width:100px;height:100px;margin-top:10px;background-color:blue;}
    <div id="d1"></div><div id="d2"></div><!--两个 div 之间的外边距是 50px,并不是 60px(50px + 10px)。-->

    例子3本身合并``它有外边距,但是没有边框或填充。在这种情况下,上外边距与下外边距就碰到了一起,它们会发生合并:

    * {margin:0; padding:0;border:0;}#d1{border: 1px solid red;width: 200px;}#d2 {height:0px;min-height:0px;margin-top:50px;margin-bottom:150px;}/*border: 1px solid red;试一试将这个盒子边框加上,是不是还能合并。答案不能!*/
    <div id="d1">    <div id="d2"></div></div>


    从图中可以看到这个红色边框包裹的高度是150px,不是200px。包含体本身木有高度,随着盒子的最后得到的折叠margin自动撑开。

    例子3调试注意

    但是在浏览器调试的时候,监视盒子的时候,并不是给我们显示折叠margin值。而是显示这个盒子的设定值。这点不要影响,最终的折叠margin值的肯定。

    例子3Question!!求答案,想了一下午。

    减轻上面的影响请试一试一下代码

    * {margin:0; padding:0;border:0;}#d2 {height:0px;min-height:0px;margin-top:50px;margin-bottom:150px;}<div id="d2"></div>

    从上面注意我们知道:调试的时候选中这个元素,显示200px是正常的,但是这个元素距离浏览器上面100px,这个我真不能理解。

    不能产生折叠边距的情况

  • BFC元素不会与兄弟,包含元素产生折叠边距。那么什么是BFC呢?敬请关注我的下一篇文章。
  • Statement
    The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
    Difficulty in updating caching of official account web pages: How to avoid the old cache affecting the user experience after version update?Difficulty in updating caching of official account web pages: How to avoid the old cache affecting the user experience after version update?Mar 04, 2025 pm 12:32 PM

    The official account web page update cache, this thing is simple and simple, and it is complicated enough to drink a pot of it. You worked hard to update the official account article, but the user still opened the old version. Who can bear the taste? In this article, let’s take a look at the twists and turns behind this and how to solve this problem gracefully. After reading it, you can easily deal with various caching problems, allowing your users to always experience the freshest content. Let’s talk about the basics first. To put it bluntly, in order to improve access speed, the browser or server stores some static resources (such as pictures, CSS, JS) or page content. Next time you access it, you can directly retrieve it from the cache without having to download it again, and it is naturally fast. But this thing is also a double-edged sword. The new version is online,

    How do I use HTML5 form validation attributes to validate user input?How do I use HTML5 form validation attributes to validate user input?Mar 17, 2025 pm 12:27 PM

    The article discusses using HTML5 form validation attributes like required, pattern, min, max, and length limits to validate user input directly in the browser.

    How to efficiently add stroke effects to PNG images on web pages?How to efficiently add stroke effects to PNG images on web pages?Mar 04, 2025 pm 02:39 PM

    This article demonstrates efficient PNG border addition to webpages using CSS. It argues that CSS offers superior performance compared to JavaScript or libraries, detailing how to adjust border width, style, and color for subtle or prominent effect

    What are the best practices for cross-browser compatibility in HTML5?What are the best practices for cross-browser compatibility in HTML5?Mar 17, 2025 pm 12:20 PM

    Article discusses best practices for ensuring HTML5 cross-browser compatibility, focusing on feature detection, progressive enhancement, and testing methods.

    What is the purpose of the <datalist> element?What is the purpose of the <datalist> element?Mar 21, 2025 pm 12:33 PM

    The article discusses the HTML <datalist> element, which enhances forms by providing autocomplete suggestions, improving user experience and reducing errors.Character count: 159

    What is the purpose of the <meter> element?What is the purpose of the <meter> element?Mar 21, 2025 pm 12:35 PM

    The article discusses the HTML <meter> element, used for displaying scalar or fractional values within a range, and its common applications in web development. It differentiates <meter> from <progress> and ex

    How do I use the HTML5 <time> element to represent dates and times semantically?How do I use the HTML5 <time> element to represent dates and times semantically?Mar 12, 2025 pm 04:05 PM

    This article explains the HTML5 <time> element for semantic date/time representation. It emphasizes the importance of the datetime attribute for machine readability (ISO 8601 format) alongside human-readable text, boosting accessibilit

    What is the purpose of the <progress> element?What is the purpose of the <progress> element?Mar 21, 2025 pm 12:34 PM

    The article discusses the HTML <progress> element, its purpose, styling, and differences from the <meter> element. The main focus is on using <progress> for task completion and <meter> for stati

    See all articles

    Hot AI Tools

    Undresser.AI Undress

    Undresser.AI Undress

    AI-powered app for creating realistic nude photos

    AI Clothes Remover

    AI Clothes Remover

    Online AI tool for removing clothes from photos.

    Undress AI Tool

    Undress AI Tool

    Undress images for free

    Clothoff.io

    Clothoff.io

    AI clothes remover

    AI Hentai Generator

    AI Hentai Generator

    Generate AI Hentai for free.

    Hot Article

    R.E.P.O. Energy Crystals Explained and What They Do (Yellow Crystal)
    2 weeks agoBy尊渡假赌尊渡假赌尊渡假赌
    Repo: How To Revive Teammates
    1 months agoBy尊渡假赌尊渡假赌尊渡假赌
    Hello Kitty Island Adventure: How To Get Giant Seeds
    4 weeks agoBy尊渡假赌尊渡假赌尊渡假赌

    Hot Tools

    SAP NetWeaver Server Adapter for Eclipse

    SAP NetWeaver Server Adapter for Eclipse

    Integrate Eclipse with SAP NetWeaver application server.

    MinGW - Minimalist GNU for Windows

    MinGW - Minimalist GNU for Windows

    This project is in the process of being migrated to osdn.net/projects/mingw, you can continue to follow us there. MinGW: A native Windows port of the GNU Compiler Collection (GCC), freely distributable import libraries and header files for building native Windows applications; includes extensions to the MSVC runtime to support C99 functionality. All MinGW software can run on 64-bit Windows platforms.

    VSCode Windows 64-bit Download

    VSCode Windows 64-bit Download

    A free and powerful IDE editor launched by Microsoft

    MantisBT

    MantisBT

    Mantis is an easy-to-deploy web-based defect tracking tool designed to aid in product defect tracking. It requires PHP, MySQL and a web server. Check out our demo and hosting services.

    mPDF

    mPDF

    mPDF is a PHP library that can generate PDF files from UTF-8 encoded HTML. The original author, Ian Back, wrote mPDF to output PDF files "on the fly" from his website and handle different languages. It is slower than original scripts like HTML2FPDF and produces larger files when using Unicode fonts, but supports CSS styles etc. and has a lot of enhancements. Supports almost all languages, including RTL (Arabic and Hebrew) and CJK (Chinese, Japanese and Korean). Supports nested block-level elements (such as P, DIV),