Then test
<html>
<head>
<style type="text/css">
body{
margin:0;
padding:0;
} #a{
margin-top:20px;
margin-bottom:20px;
}
#b{
width:100px;
height:50px;
background:green;
}
</style> <head>
<body> <div id="a"></div>
<div id="b"></div> </body>
</html>
This IE6 is the same as Firefox. Note that the green top is not 40px but 20px
Margin merging will only occur for the vertical margins of block boxes in normal document flow. Margins between inline boxes, floated boxes, or absolutely positioned boxes are not merged.
Let’s talk about document flow. Only when you know the concept of flow can you talk about positioning
CSS has three basic positioning mechanisms: ordinary flow, floating and absolute positioning
Unless specifically specified, all boxes are Locate in the normal stream. That is, the position of an element in the normal flow is determined by the element's position in (X)HTML. Block-level boxes are arranged one after another from top to bottom, and the vertical distance between boxes is calculated from the vertical margins of the boxes. Inline boxes are arranged horizontally in a row. Their spacing can be adjusted using horizontal padding, borders, and margins. However, vertical padding, borders, and margins do not affect the height of the inline box. The horizontal box formed by a line is called a line box. The height of a line box is always high enough to accommodate all the inline boxes it contains. However, setting the row height can increase the height of this box.
The flow is like running water, from top to bottom, from left to right, just like a word document. If you insert a text in the middle of the text, the following text will be squeezed out automatically. The same is true for the normal layout of the web page
Let’s talk about the concept of block first
div, h1 or p elements are often called block-level elements. This means that these elements appear as a block of content, a "block box". In contrast, elements such as span and strong are called "inline elements" because their content appears within a line, an "inline box."
To put it simply, if you do not specify the width of the block element, its default width will be 100%, that is, the width of the browser will be the same, and a
will be automatically added at the end, so there are two Even if you set the width of the divs to 100px, they will not be horizontal. The first div will force the second div to wrap, and the second div will force the third div to wrap. . . .
Inline elements have no width and height. They are just one line. If you directly specify the width and height with CSS, it is invalid, but its width and height can only change with the content.
Blocks and inline elements can both become each other. Use the display attribute
Let’s talk about float floating
First make a big box
<html>
<head>
<style type="text/css">
body{
margin:0;
padding:0;
} #a{
width:500px;
height:500px;
border:solid;
}
.div{
width:100px;
height:100px;
background:green;
}
</style>
<head>
<body>
<div id="a"> </div>
</body>
</html>
We know that if there are 2 in the big box div, then these two divs will not be arranged horizontally
<html>
<head>
<style type="text/css">
body{
margin:0;
padding:0;
} #a{
width:500px;
height:500px;
border:solid;
}
.div{
width:100px;
height:100px;
border:soild;
margin:1px;
background:green;
}
</style>
<head>
<body>
<div id="a">
<div class="div"></div>
<div class="div"></div>
</div>
</body>
</html>
If you want them to be arranged horizontally, you have to use float
<html>
<head>
<style type="text/css">
body{
margin:0;
padding:0;
} #a{
width:500px;
height:500px;
border:solid;
}
.div{
width:100px;
height:100px;
border:soild;
margin:1px;
background:green;
float:left;
}
</style>
<head>
<body>
<div id="a">
<div class="div"></div>
<div class="div"></div>
</div>
</body>
</html>
如果第2个div不设置float会怎样？
<html>
<head>
<style type="text/css">
body{
margin:0;
padding:0;
} #a{
width:500px;
height:500px;
border:solid;
}
.div{
width:100px;
height:100px;
border:soild;
margin:1px;
background:green;
float:left;
}
#b{
width:100px;
height:100px;
border:soild;
margin:1px;
background:green;
}
</style>
<head>
<body>
<div id="a">
<div class="div"></div>
<div id="b"></div>
</div>
</body>
</html>
You can see that it is still the same, but the middle spacing is 2px, which means that the horizontal margins of the two divs will not overlap.
The vertical ones will. The specifics need to be verified through practice
At this time, I am looking at the Firefox browser
Why is there only one left? In fact, the two divs overlap.
Which browser depends on it? Of course, it is based on Firefox.
The floating box can move to the left or right until its outer edge touches the containing box or to the border of another floating box. Because the floated box is not in the document's normal flow, a block box in the document's normal flow behaves as if the floated box does not exist.
How to understand this sentence?
You can think of this big box as an airport, and the div is an airplane.
The div without float is the airplane parked on the ground, and the div with float is the airplane in the air.
The div with float will not occupy it. The space of the div on the ground, so they overlap, which is equivalent to saying that one is in the sky and the other is on the ground. This is why it is called floating, because it is a div floating in the air
The above is a new beginning of div+css web layout design ( 6), please pay attention to the PHP Chinese website (www.php.cn) for more related content!
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