どうぞ :-
マージン崩壊
隣接する兄弟
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>
親と最初/最後の子
If there is no border, padding, inline content, or clearance to separate the margin-top of a block with the margin-top of its first child block, or no border, padding, inline content, height, min-height, or max-height to separate the margin-bottom of a block with the margin-bottom of its last child, then those margins collapse. The collapsed margin ends up outside the parent.
空のブロック
If there is no border, padding, inline content, height, or min-height to separate a block's margin-top from its margin-bottom, then its top and bottom margins collapse.
このフィドルを見てください-スパンをいくつか追加しただけで、今は崩壊しません。
http://jsfiddle.net/aPaBy/
<div style="background-color: red; border-style:solid;">
<div id="test" class="border" style="background-color: yellow;">
<span>hi</span>
<p>Inside <button>Test</button></p>
<span>hi</span>
</div>
<p>Something</p>
</div>