边境财产对最高利润的影响

问题描述

请参阅以下代码:

body {
  height: 500px;
  width: 80%;
  margin-top: 0;
  margin-bottom: 0;
  margin-left: auto;
  margin-right: auto;
  padding: 0;
  background-color: lightgray;
}
.header {
  width: 80%;
  height: 100px;
  margin-left: auto;
  margin-right: auto;
  background-color: yellow;
  /* border: solid 1px black; */
}

<div class="header">
  <ul>
    <li><a href="index.html">Dashboard</a></li>
  </ul>
</div>

当我从.header中删除border: solid 1px black时，我在div元素上方获得了一些空间.但是，当我添加border属性时，我得到了完美的结果(即删除了空格).边界财产对该空间有何影响?

When I remove border: solid 1px black from .header I get some space above div element. But when I add the border property I get the perfect results (that is space removed). What should be impact of border property on this space?

推荐答案

这是由于页边空白( MDN ， W3. org ).

This is due to margin collapsing (MDN, W3.org).

在您的示例中，header元素包含一个无序列表，默认情况下，该列表的顶部和底部都有页边距.由于边界折叠，它的边距被传输到页眉，而页眉又被传输到主体.结果，当标题和列表与主体的顶部对齐时，主体被向下推.

In your example the header element contains an unordered list which, by default, has a top and bottom margin. Because of border collapsing, its margin is transferred to the header which in turn is transferred to the body. As a result, the body is pushed down while the header and list aligns with the top of body.

添加边框是防止边距塌陷的一种方法(请参阅W3规格).如果要避免添加边框，请使用其他方法，例如将overflow: hidden分配给标题.

Adding a border is one method to prevent margin collapsing (see W3 specs). If you want to avoid adding a border, use the other methods such as assign overflow: hidden to the header.

body {
  height: 500px;
  width: 80%;
  margin-top: 0;
  margin-bottom: 0;
  margin-left: auto;
  margin-right: auto;
  padding: 0;
  background-color: lightgray;
}
.header {
  width: 80%;
  height: 100px;
  margin-left: auto;
  margin-right: auto;
  background-color: yellow;
  /*border: solid 1px black;  */
  overflow: hidden;
}

<div class="header">
  <ul>
    <li><a href="index.html">Dashboard</a></li>
  </ul>
</div>

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