Margin on child element moves parent element

user_75642 0
Viewed 211749

I have a div (parent) that contains another div (child). Parent is the first element in body with no particular CSS style. When I set

.child
{
    margin-top: 10px;
}

The end result is that top of my child is still aligned with parent. Instead of child being shifted for 10px downwards, my parent moves 10px down.

My DOCTYPE is set to XHTML Transitional.

What am I missing here?

edit 1
My parent needs to have strictly defined dimensions because it has a background that has to be displayed under it from top to bottom (pixel perfect). So setting vertical margins on it is a no go.

edit 2
This behaviour is the same on FF, IE as well as CR.

xhtml margin css nested
15 Answers

Found an alternative at Child elements with margins within DIVs You can also add:

.parent { overflow: auto; }

or:

.parent { overflow: hidden; }

This prevents the margins to collapse. Border and padding do the same. Hence, you can also use the following to prevent a top-margin collapse:

.parent {
    padding-top: 1px;
    margin-top: -1px;
}

2021 update: if you're willing to drop IE11 support you can also use the new CSS construct display: flow-root. See MDN Web Docs for the whole details on block formatting contexts.

Update by popular request: The whole point of collapsing margins is handling textual content. For example:

h1, h2, p, ul {
  margin-top: 1em;
  margin-bottom: 1em;
  outline: 1px dashed blue;
}

div { outline: 1px solid red; }
<h1>Title!</h1>
<div class="text">
  <h2>Title!</h2>
  <p>Paragraph</p>
</div>
<div class="text">
  <h2>Title!</h2>
  <p>Paragraph</p>
  <ul>
    <li>list item</li>
  </ul>
</div>

Because the browser collapses margins, the text would appear as you'd expect, and the <div> wrapper tags don't influence the margins. Each element ensures it has spacing around it, but spacing won't be doubled. The margins of the <h2> and <p> won't add up, but slide into each other (they collapse). The same happens for the <p> and <ul> element.

Sadly, with modern designs this idea can bite you when you explicitly want a container. This is called a new block formatting context in CSS speak. The overflow or margin trick will give you that.

user_1462890

Neat CSS-only solution

Use the following code to prepend a contentless first-child to the unintentionally moving div:

.parent:before
{content: '';position: relative;height: 0px;width: 0px;overflow: hidden;white-space: pre;}

The advantage of this method is that you do not need to change the CSS of any existing element, and therefore has minimal impact on design. Next to this, the element that is added is a pseudo-element, which is not in the DOM-tree.

Support for pseudo-elements is wide-spread: Firefox 3+, Safari 3+, Chrome 3+, Opera 10+, and IE 8+. This will work in any modern browser (be careful with the newer ::before, which is not supported in IE8).

Context

If the first child of an element has a margin-top, the parent will adjust its position as a way of collapsing redundant margins. Why? It's just like that.

Given the following problem:

<style type="text/css">
div {position: relative;}
.parent {background-color: #ccc;}
.child {margin-top: 40px;}
</style>

<div class="parent"><!--This div moves 40px too-->
    <div class="child">Hello world!</div>
</div>

You can fix it by adding a child with content, such as a simple space. But we all hate to add spaces for what is a design-only issue. Therefore, use the white-space property to fake content.

<style type="text/css">
div {position: relative;}
.parent {background-color: #ccc;}
.child {margin-top: 40px;}
.fix {position: relative;white-space: pre;height: 0px;width: 0px;overflow: hidden;}
</style>

<div class="parent"><!--This div won't move anymore-->
    <div class="fix"></div>
    <div class="child">Hello world!</div>
</div>

Where position: relative; ensures correct positioning of the fix. And white-space: pre; makes you not having to add any content - like a white space - to the fix. And height: 0px;width: 0px;overflow: hidden; makes sure you'll never see the fix.

You might need to add line-height: 0px; or max-height: 0px; to ensure the height is actually zero in ancient IE browsers (I'm unsure). And optionally you could add <!--dummy--> to it in old IE browsers, if it does not work.

In short, you can do all this with only CSS (which removes the need to add an actual child to the HTML DOM-tree):

<style type="text/css">
div {position: relative;}
.parent {background-color: #ccc;}
.child {margin-top: 40px;}

.parent:before {content: '';position: relative;height: 0px;width: 0px;overflow: hidden;white-space: pre;}
</style>

<div class="parent"><!--This div won't move anymore-->
    <div class="child">Hello world!</div>
</div>
user_10099010

Play with display of parent

.parent{
     display: inline-block;
     width: 100%;
}

or

.parent{ display: flex; }
user_152649450
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