我有一个问题，居中有一个元素的属性位置设置为绝对。
有人知道为什么这些图像没有居中吗?
body {
text-align: center;
}
#slideshowWrapper {
margin-top: 50px;
text-align: center;
}
ul#slideshow {
list-style: none;
position: relative;
margin: auto;
}
ul#slideshow li {
position: absolute;
}
ul#slideshow li img {
border: 1px solid #ccc;
padding: 4px;
height: 450px;
}
<body>
<div id="slideshowWrapper">
<ul id="slideshow">
<li><img src="https://source.unsplash.com/random/300*300?technology" alt="Dummy 1" /></li>
<li><img src="https://source.unsplash.com/random/301*301?technology" alt="Dummy 2" /></li>
</ul>
</div>
</body>
要居中一个位置:绝对属性，你需要设置left:50%和margin-left: -50%的div宽度。
<!-- for horizontal -->
<style>
div.center{
width:200px;
left:50%;
margin-left:-100px;
position:absolute;
}
</style>
<body>
<div class='center'>
should be centered horizontaly
</div>
</body>
对于垂直中心绝对，你需要做同样的事情，而不是用左边，只是用顶部。
(注意:html和body必须有min-height 100%;)
<!-- for vertical -->
<style>
body,html{
min-height:100%;
}
div.center{
height:200px;
top:50%;
margin-top:-100px;
position:absolute;
}
</style>
<body>
<div class='center'>
should be centered verticaly
</div>
</body>
并且可以两者结合使用
<!-- for both -->
<style>
body,html{
min-height:100%;
}
div.center{
width:200px;
height:50px
left:50%;
top:50%;
margin-left:-100px;
margin-top:-25px;
position:absolute;
}
</style>
<body>
<div class='center'>
should be centered
</div>
</body>
#parent
{
position : relative;
height: 0;
overflow: hidden;
padding-bottom: 56.25% /* images with aspect ratio: 16:9 */
}
img
{
height: auto!important;
width: auto!important;
min-height: 100%;
min-width: 100%;
position: absolute;
display: block;
/* */
top: -9999px;
bottom: -9999px;
left: -9999px;
right: -9999px;
margin: auto;
}
我不记得我在哪里看到了上面列出的居中方法，使用负的上、右、下、左值。
对我来说，在大多数情况下，这个技巧是最好的。
当我使用上面的组合时，图像的行为就像一个背景图像，具有以下设置:
background-position: 50% 50%;
background-repeat: no-repeat;
background-size: cover;
关于第一个例子的更多细节可以在这里找到:
使用CSS维护div的纵横比
我不确定你想要完成什么，但在这种情况下，只是增加宽度:100%;到你的ul#slideshow li将做的把戏。
解释
The img tags are inline-block elements. This means that they flow inline like text, but also have a width and height like block elements. In your css there are two text-align: center; rules applied to the <body> and to the #slideshowWrapper (which is redundant btw) this makes all inline and inline-block child elements to be centered in their closest block elements, in your code these are li tags. All block elements have width: 100% if they are the static flow (position: static;), which is default. The problem is that when you tell li tags to be position: absolute;, you take them out of normal static flow, and this causes them to shrink their size to just fit their inner content, in other words they kind of "lose" their width: 100% property.
目前似乎有两种解决方案;边距居中，位置居中。这两种工作都很好，但是如果您想要相对于这个居中元素的绝对位置，则需要使用绝对位置方法，因为第二个元素的绝对位置默认为所定位的第一个父元素。像这样:
<!-- CENTERED USING MARGIN -->
<div style="width:300px; height:100px; border: 1px solid #000; margin:20px auto; text- align:center;">
<p style="line-height:4;">width: 300 px; margin: 0 auto</p>
<div style="position:absolute; width:100px; height:100px; background-color:#ff0000; top:-20px; left:0px;">
<p style="line-height:4;">Absolute</p>
</div>
</div>
<!-- CENTERED USING POSITION -->
<div style="position:absolute; left:50%; width:300px; height:100px; border: 1px solid #000; margin:20px 0 20px -150px; text-align:center;">
<p style="line-height:2;">width:300px; position: absolute; left: 50%; margin-left:-150px;</p>
<div style="position:absolute; width:100px; height:100px; background-color:#ff0000; top:0px; left:-105px;">
<p style="line-height:4;">Absolute</p>
</div>
</div>
直到我读到这篇文章，使用边距:0自动技术，在我的内容的左边建立一个菜单，我必须在右边建立一个相同宽度的列来平衡它。不漂亮。谢谢!
