英文教程:CSS的Float

Clearing the Float

Float’s sister property is clear. An element that has the clear property set on it will not move up adjacent to the float like the float desires, but will move itself down past the float. Again an illustration probably does more good than words do.

In the above example, the sidebar is floated to the right and is shorter than the main content area. The footer then is required to jump up into that available space as is required by the float. To fix this problem, the footer can be cleared to ensure it stays beneath both floated columns.

#footer {
   clear: both;
}

Clear has four valid values as well. Both is most commonly used, which clears floats coming from either direction. Left and Right can be used to only clear the float from one direction respectively. None is the default, which is typically unnecessary unless removing a clear value from a cascade. Inherit would be the fifth, but is strangly not supported in Internet Explorer. Clearing only the left or right float, while less commonly seen in the wild, definitely has its uses.

The Great Collapse

One of the more bewildering things about working with floats is how they can affect the element that contains them (their “parent” element). If this parent element contained nothing but floated elements, the height of it would literally collapse to nothing. This isn’t always obvious if the parent doesn’t contain any visually noticeable background, but it is important to be aware of.

As anti-intuitive as collapsing seems to be, the alternative is worse. Consider this scenario:

If the block element on top where to have automatically expanded to accomodate the floated element, we would have an unnatural spacing break in the flow of text between paragraphs, with no practica

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