Vertical margins disappear when parent is set to overflow:visible

Question

Why do vertical margins disappear when the parent is set to overflow:visible? If it's set to overflow:hidden margins are visible again. It seems counterintuitive.

I think I understand how overflow is supposed to work when content of an element can't fit into it, but I don't understand what is happening with the margins.

Here's an example: ( http://jsfiddle.net/VrVc7/ )

#outer {
    background-color:#EEE;
    overflow:hidden;
}

#inner {
    margin: 30px;
    padding: 5px;
    background-color:#ABE;
}

<div id="outer">
    <div id="inner">abc</div>
</div>

Answer 1

It's called collapsing margin. As per W3c

In CSS, the adjoining margins of two or more boxes (which might or might not be siblings) can combine to form a single margin. Margins that combine this way are said to collapse, and the resulting combined margin is called a collapsed margin.

How to prevent from collapsing margin.

1. Margins between a floated box and any other box do not collapse (not even between a float and its in-flow children).
2. Margins of elements that establish new block formatting contexts (such as floats and elements with 'overflow' other than 'visible') do not collapse with their in-flow children.
3. Margins of absolutely positioned boxes do not collapse (not even with their in-flow children).
4. Margins of inline-block boxes do not collapse (not even with their in-flow children).
5. The bottom margin of an in-flow block-level element always collapses with the top margin of its next in-flow block-level sibling, unless that sibling has clearance.
6. The top margin of an in-flow block element collapses with its first in-flow block-level child's top margin if the element has no top border, no top padding, and the child has no clearance.
7. The bottom margin of an in-flow block box with a 'height' of 'auto' and a 'min-height' of zero collapses with its last in-flow block-level child's bottom margin if the box has no bottom padding and no bottom border and the child's bottom margin does not collapse with a top margin that has clearance.
8. A box's own margins collapse if the 'min-height' property is zero, and it has neither top or bottom borders nor top or bottom padding, and it has a 'height' of either 0 or 'auto', and it does not contain a line box, and all of its in-flow children's margins (if any) collapse.