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Top and bottom margins of blocks are sometimes combined (collapsed) into a single margin whose size is the largest of the margins (if the elements had equal margin it will take one of them, it doesn't sum them together) combined into it, a behavior known as margin collapsing.
Margin collapsing occurs in three basic cases:
- Adjacent siblings
- The margins of adjacent siblings are collapsed (except when the later sibling needs to be cleared past floats). For example:
<p>The bottom margin of this paragraph is collapsed...</p> <p>...with the top margin of this paragraph.</p>
- Parent and first/last child
- If there is no border, padding, inline part,
block_formatting_contextcreated or clearance to separate the
margin-topof a block from the
margin-topof its first child block, or no border, padding, inline content,
max-heightto separate the
margin-bottomof a block from the
margin-bottomof its last child, then those margins collapse. The collapsed margin ends up outside the parent.
- Empty blocks
- If there is no border, padding, inline content,
min-heightto separate a block's
margin-bottom, then its top and bottom margins collapse.
More complex margin collapsing (of more than two margins) occurs when these cases are combined.
These rules apply even to margins that are zero, so the margin of a first/last child ends up outside its parent (according to the rules above) whether or not the parent's margin is zero.
When negative margins are involved, the size of the collapsed margin is the sum of the largest positive margin and the smallest (most negative) negative margin.
If both are negative, the greater negative value is used. This definition applies to adjacent elements and nested elements.
|CSS Level 2 (Revision 1)
The definition of 'margin collapsing' in that specification.
- CSS Reference
- CSS Key Concepts: CSS syntax, at-rule, comments, specificity and inheritance, the box, layout modes and visual formatting models, and margin collapsing, or the initial, computed, resolved, specified, used, and actual values. Definitions of value syntax, shorthand properties and replaced elements.