Summary of HTML horizontal and vertical centering issues

I have encountered many centering problems recently, so I took some time to summarize them and put them here so that it will be easy to find them later.

1. Center text

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The code is as follows:

<div class="wrap">
I'm in the middle...
</div>
.. height+line-height+text-center (can only center a single line)
.wrap{
width:px;
height:px;
border:px solid red;
text-align: center;
line-height: px;
}

ps:text-align:center only centers the inline elements below the element
1.2display:table-cell (multiple rows with fixed height centered)

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The code is as follows:

.wrap{
width:px;
height:px;
border:px solid red;
text-align: center;
display:table-cell;
vertical-align: middle;
}

display:table-cell: does not work in ie67, it is best to use it with display:table;
Under ie67: (I won't use it anymore, but I'll put it here)
Method 1: (The em tag height is the same as the parent, so span and em centering is equivalent to span centering the parent)

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The code is as follows:

<div class="wrap">
<span>
I'm in the middle... I'm in the middle... I'm in the middle...
</span>
<em>
</div>
.wrap{
width:px;
height:px;
border:px solid red;
text-align: center;
}
.wrap span{
vertical-align: middle;
display:inline-block;
width:px;
}
.wrap em{
height:%;
vertical-align: middle;
display:inline-block;
}

Method 2: (By adding an absolutely positioned parent tag to the child element, and then using relative positioning of the child element to center it horizontally and vertically)

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The code is as follows:

<div class="wrap">
<span class="span">
<span class="span">I'm in the middle... I'm in the middle... I'm in the middle... I'm in the middle...</span>
</span>
</div>
.wrap{
width:px;
height:px;
border:px solid red;
display:table;
position:relative;
overflow: hidden;
}
.wrap .span{
display:table-cell;
vertical-align: middle;
text-align: center;
*position:absolute;
top:%;
left:%;
}
.wrap .span{
*position:relative;
top:-%;
left:-%;
}

1.3padding (internal padding, needless to say)

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The code is as follows:

.wrap{
width:px;
border:px solid red;
padding:px;
}

2. Centering Elements

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The code is as follows:

2.1position:absolute+margin negative value (width and height are required to calculate margin)

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The code is as follows:

.wrap{
width:px;
height:px;
position:absolute;
top:%;
left:%;
margin-top:-px;
margin-left:-px;
border:px solid red;
}
.wrap span{
width:px;
height:px;
background:red;
position: absolute;
top:%;
left:%;
margin-top:-px;
margin-left:-px;
}

ps: CSS realizes DIV horizontal centering and vertical centering

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The code is as follows:

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>Vertical centering online demonstration DIVCSS5</title>
<style>
#main {
position: absolute;
width:400px;
height:200px;
left:50%;
top:50%;
margin-left:-200px;
margin-top:-100px;
border:1px solid #00F
}
/*css comment: For the convenience of screenshots, wrap the CSS code*/
</style>
</head>
<body>
<div id="main">DIV horizontal center and vertical center<a href="http://www.divcss5.com/">DIVCSS5</a></div>
</body>
</html>

Introduction to the principles of horizontal and vertical centering <br />Absolute positioning position:absolute is used here, and left and top are used to set the object's distance from the top and left to 50%. However, if it is set to 50%, the box is actually not centered, so margin-left:-200px;margin-top:-100px; are set. Here is a trick: the value of margin-left is half of the width, and the value of margin-top is also half of the object height. At the same time, they are set to negative, so that horizontal and vertical centering is achieved.

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