Why does this CSS margin-top style not work?

You're actually seeing the top margin of the #inner element collapse into the top edge of the #outer element, leaving only the #outer margin intact (albeit not shown in your images). The top edges of both boxes are flush against each other because their margins are equal.

Here are the relevant points from the W3C spec:

8.3.1 Collapsing margins 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. Adjoining vertical margins collapse [...]

Two margins are adjoining if and only if: both belong to in-flow block-level boxes that participate in the same block formatting context no line boxes, no clearance, no padding and no border separate them both belong to vertically-adjacent box edges, i.e. form one of the following pairs: top margin of a box and top margin of its first in-flow child

You can do any of the following to prevent the margin from collapsing:

Float either of your div elements Make either of your div elements inline blocks Set overflow of #outer to auto (or any value other than visible)

The reason the above options prevent the margin from collapsing is because:

Margins between a floated box and any other box do not collapse (not even between a float and its in-flow children). 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. Margins of inline-block boxes do not collapse (not even with their in-flow children).

The left and right margins behave as you expect because:

Horizontal margins never collapse.

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Try using display: inline-block; on the inner div. Like so:

#outer {
    width:500px; 
    height:200px; 
    background:#FFCCCC;
    margin:50px auto 0 auto;
    display:block;
}
#inner {
    background:#FFCC33;
    margin:50px 50px 50px 50px;
    padding:10px;
    display:inline-block;
}

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What @BoltClock mentioned are pretty solid. And Here I just want to add several more solutions for this problem. check this w3c_collapsing margin. The green parts are the potential thought how this problem can be solved.

Solution 1

Margins between a floated box and any other box do not collapse (not even between a float and its in-flow children).

that means I can add float:left to either #outer or #inner demo1.

also notice that float would invalidate the auto in margin.

Solution 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.

other than visible, let's put overflow: hidden into #outer. And this way seems pretty simple and decent. I like it.

#outer{
    width: 500px;
    height: 200px;
    background: #FFCCCC;
    margin: 50px auto;
    overflow: hidden;
}
#inner {
    background: #FFCC33;
    height: 50px;
    margin: 50px;
}

Solution 3

Margins of absolutely positioned boxes do not collapse (not even with their in-flow children).

#outer{
    width: 500px;
    height: 200px;
    background: #FFCCCC;
    margin: 50px auto;
    position: absolute; 
}
#inner{
    background: #FFCC33;
    height: 50px;
    margin: 50px;
}

or

#outer{
    width: 500px;
    height: 200px;
    background: #FFCCCC;
    margin: 50px auto;
    position: relative; 
}
#inner {
    background: #FFCC33;
    height: 50px;
    margin: 50px;
    position: absolute;
}

these two methods will break the normal flow of div

Solution 4

Margins of inline-block boxes do not collapse (not even with their in-flow children).

is the same as @enderskill

Solution 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.

This has not much work to do with the question since it is the collapsing margin between siblings. it generally means if a top-box has margin-bottom: 30px and a sibling-box has margin-top: 10px. The total margin between them is 30px instead of 40px.

Solution 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.

This is very interesting and I can just add one top border line

#outer{
    width: 500px;
    height: 200px;
    background: #FFCCCC;
    margin: 50px auto;
    border-top: 1px solid red;

}
#inner {
    background: #FFCC33;
    height: 50px;
    margin: 50px;

}

And Also <div> is block-level in default, so you don't have to declare it on purpose. Sorry for not being able to post more than 2 links and images due to my novice reputation. At least you know where the problem comes from next time you see something similar.

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Not sure why what you have doesn't work, but you can add overflow: auto; to the outer div.

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Not exactly sure why, but changing the inner CSS to

display: inline-block;

seems to work.

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If you add any padding to #outer, it works. Demo here:

#outer { width:500px; height:200px; background:#FFCCCC; margin:50px auto 0 auto; display:block; padding-top:1px; } #inner { background:#FFCC33; margin:50px 50px 50px 50px; padding:10px; display:block; }

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Doesn't answer the "why" (has to be something w/ collapsing margin), but seems like the easiest/most logical way to do what you're trying to do would be to just add padding-top to the outer div:

#outer { width:500px; height:200px; background:#FFCCCC; margin:50px auto 0 auto; padding-top: 50px; } #inner { background:#FFCC33; margin:0px 50px 50px 50px; padding:10px; }

Minor note - it shouldn't be necessary to set a div to display:block; unless there's something else in your code telling it not to be block.

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Create new block formatting context

You can use display: flow-root on the parent element to prevent margin collapsing through the containing element as it creates new Block Formatting Context.

Changing the value of the overflow property to auto or using flexbox will have the same effect.

https://codepen.io/rachelandrew/pen/VJXjEp

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Try this:

#outer { width:500px; height:200px; background:#FFCCCC; margin:50px auto 0 auto; display:table; } #inner { background:#FFCC33; margin:50px 50px 50px 50px; padding:10px; display:block; }

Good luck!

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I guess setting the position property of the #inner div to relative may also help achieve the effect. But anyways I tried the original code pasted in the Question on IE9 and latest Google Chrome and they already give the desirable effect without any modifications.

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Use padding-top:50pxfor outer div. Something like this:

#outer {
    width:500px; 
    height:200px; 
    background:#FFCCCC;
    margin:50px auto 0 auto;
    display:table;}

Note: padding will increase the size of your div. In this case if the size of your div is important, I mean if it must have a specific height. decrease the height by 50px.:

#outer {
    width:500px; 
    height:150px; 
    background:#FFCCCC;
    margin:50px auto 0 auto;
    display:table;}

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Have you tried !important before all, it will force everything:

margin:50px 50px 50px 50px !important;

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If you have a margin collapse issue, then to resolve this you can add display: flow-root; to the parent container.

Aside from that, if margin-top is being ignored, try margin-top with a negative value, for instance: margin-top: -2px;

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Just for a quick fix, try wrapping your child elements into a div element like this -

<div id="outer">
   <div class="divadjust" style="padding-top: 1px">
      <div id="inner">
         Hello world!
      </div>
   </div>
</div>

Margin of inner div won't collapse due to the padding of 1px in-between outer and inner div. So logically you will have 1px extra space along with existing margin of inner div.

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