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CSS about clearfix clearing floating method

高洛峰

Mar 17, 2017 pm 04:26 PM

cssclear float

1. What is .clearfix

As long as you search for "css clear float" on Google or Baidu, you will find that many websites talk about "when the box clears internal floats" You can use .clearfix".

.clearfix:after {
  content: " ";
  display: block;
  clear: both;
  height: 0;
}
.clearfix {
  zoom: 1;
}<p class="clearfix">
  <p class="floated"></p>
</p>

The above code is the definition and application of .clearfix. Let’s briefly talk about the principle of .clearfix:

1. In IE6 and 7, zoom: 1 will trigger hasLayout, thus making the element Close the inner float.

2. Under standard browsers, the .clearfix:after pseudo-class will insert a clear: both block-level element after the element applied to .clearfix, thereby clearing the float.

<p>
  <p class="floated"></p>
</p>
<p style="clear: both"></p>

2. The disadvantages of .clearfix

As you can see in the above code, leaving aside IE6 and 7, .clearfix is inserted under a standard browser An element with clear: both is added, which is likely to clear unnecessary floats. To illustrate with an example:

<!DOCTYPE html>
<html>
<head>
  <title>Demo</title>
  <style type="text/css">
    html, body { padding: 0; margin: 0; }
    ul { margin: 0; padding: 0; } 
  
    .clearfix:after {
      content: " ";
      display: block;
      clear: both;
      height: 0;
    }
    .clearfix {
      zoom: 1;
    }
  
    .left-col {
      background: red;      float: left;
      width: 100px;
      height: 300px;
    }
    .right-col {
      margin-left: 100px;
    }
    .menu {
      border: 1px solid #000;
    }
    .menu li {      float: left;
      display: block;
      padding: 0 1em;
      margin: 0 1em 0 0;
      background: #ccc;
    }
    .placeholder {
      background: yellow;
      height: 400px;
    }  </style>
</head>
<body>
  <p class="left-col">
  </p>
  <p class="right-col">
    <ul class="menu">
      <li>Menu Item</li>
      <li>Menu Item</li>
      <li>Menu Item</li>
      <li>Menu Item</li>
      <li>Menu Item</li>
      <li>Menu Item</li>
    </ul>
    <p class="placeholder"></p>
  </p>
</body>
</html>

The above code constitutes a two-column layout page. Note that the .menu menu has a border, but because the li element of the .menu floats left, the .menu has no height, so you can use .clearfix to clear the floating content inside the .menu. The code is as follows:

<ul class="menu clearfix">
  <li>Menu Item</li>
  <li>Menu Item</li>
  <li>Menu Item</li>
  <li>Menu Item</li>
  <li>Menu Item</li>
  <li>Menu Item</li>
</ul>

But after applying .clearfix, the page becomes very messy under the standard browser. This is because .clearfix:after also clears the float of .left-col.

3. Reconstruct .clearfix

After encountering the above error, analyze whether there is any other method besides .clearfix:after. Clear the float inside the element. The answer is yes. In the article Block Formatting Contexts in vernacular, it is mentioned that the elements that constitute the Block Formatting Context can clear the floating of internal elements. Then just make .clearfix form a Block Formatting Context. There are several methods to construct Block Formatting Context:

The value of float is not none.
The value of overflow is not visible.
The value of display is any one of table-cell, table-caption, and inline-block.
The value of position is not relative or static.

Obviously, float and position are not suitable for our needs. Then you can only choose one from overflow or display. Because the menu that applies .clearfix and .menu is most likely to be multi-level, overflow: hidden or overflow: auto does not meet the needs (it will hide the drop-down menu or display the scroll bar), so you can only use display Take action.

We can set the display value of .clearfix to any one of table-cell, table-caption, and inline-block, but display: inline-block will produce redundant blanks, so it is also excluded. The only ones left are table-cell and table-caption. In order to ensure compatibility, you can use display: table to make .clearfix form a Block Formatting Context, because display: table will generate some anonymous boxes. One of these anonymous boxes (the display value is table-cell) will form a Block Formatting Context. In this way, our new .clearfix will close the float of the inner element. Below is the .clearfix after refactoring.

.clearfix {
  zoom: 1;
  display: table;
  width: 100%;
}

Four, summary

Under IE6 and 7, as long as the element that triggers hasLayout can clear the internal float. There are many ways to clear internal floats of elements under standard browsers. Except for .clearfix:after, the other methods are nothing more than generating a new Block Formatting Context to achieve the purpose. If you can use which method under what conditions, I think this is enough...

For more CSS related articles about the clearfix clearing floating method, please pay attention to the PHP Chinese website!

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