Problems and Countermeasures: CSS margin collapse_html/css

Problems and Countermeasures: CSS margin collapse_html/css_WEB-ITnose

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Release： 2016-06-24 11:59:13

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   2: <html>

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   3: <head>

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   4:     <title></title>

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   5:     <metaname=""content="">

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   6: <style>

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   7: *{

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   8:     margin:0px;

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   9:     padding:0px;

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  10: }

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11:

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  12: #no1{

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  13:     background:#808000;

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  14:     width:300px;

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  15:     height:300px;

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  16:     margin:100px 0 0 100px;

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  17:     }

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18:

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  19: #no2{

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  20:     background:#c0c0c0;

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  21:     width:150px;

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  22:     height:150px;

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  23:     margin-left:20px;

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  24:     margin-top:30px;

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  25: }

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  26: </style>

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  27: </head>

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28:

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  29: <body>

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30:

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  31: <div id="no1">

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  32:     <div id="no2">Span2</div>

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  33: </div>

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34:

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  35: </body>

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36:

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  37: </html>

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If you think it should be like this:

That’s wrong. The result is this:

Because there is a margin collapse in CSS, that is, border collapse or border overlap. For two juxtaposed div blocks, the margin-bottom of the upper div and the margin-top of the lower div will collapse, that is, the maximum value of the upper and lower margins will be taken as the display value, so in this sense: CSS and browser designers hope that when we encounter an arrangement of two side-by-side content blocks, it is best to only set the upper or lower margin of each block . w3school also stipulates: When two vertical margins meet, they will form a margin. The height of the merged margin is equal to the greater of the heights of the two merged margins.

But for the case where the parent block DIV contains the child block DIV, it will be explained according to another CSS convention, that is: For elements with block-level child elements, the height is calculated In this way, if the element has no vertical border and padding, its height is the distance between the top and bottom border edges of its child elements. So for the code:

   1: <div class="father"></div>

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The height of the father div is 0 because there is no content in it that can open the div. If it changes to:

   1: <div class="father">I am here.</div>

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, then the height is the height of the text, because the text is supporting the DIV at this time.

Having said that, a DIV and its sub-DIVs pay special attention to the vertical border or padding, which is like a pot with a basin inside. Whether it can hold the basin inside mainly depends on the lid. The vertical border or padding is this "lid." So there are at least three solutions:

1. You need to set a border for the parent div. Of course, you can set the border to be transparent:

   1: border:1px solid transparent

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   2: 或

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   3: border-top:1px solid transparent

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2. Add padding to the parent DIV, or at least add padding-top;

   1: padding:1px

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   2: 或

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   3: padding-top:1px;   /*必须大于0*/

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3. Add overflow:hidden to the parent DIV

   1: over-flow:hidden;

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Source: http://www.ido321.com/387.html