Margin-Top push outer div down

Question

put overflow:auto in the parent div
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I don't have a solid explanation on why this happens, but I've fixed this by changing the top-margin to top-padding, or adding display: inline-block to the element style.

EDIT: This is my theory

I have a feeling it has something to do with how margins are collapsed (combined).

from W3C Collapsing Margins:

In this specification, the expression collapsing margins means that adjoining margins (no non-empty content, padding or border areas or clearance separate them) of two or more boxes (which may be next to one another or nested) combine to form a single margin.

My theory is that since your first element is next to the body the two margins combine and are applied to the body: this forces the body's content to start below the applied collapsed margin in compliance with the box model.

There are situations where the margins do not collapse which might be worth playing around with (from Collapsing Margins):

* floated elements
* absolutely positioned elements
* inline-block elements
* elements with overflow set to anything other than visible (They do not collapse margins with their children.)
* cleared elements (They do not collapse their top margins with their parent block’s bottom margin.)
* the root element

This are some of the ways to avoid margin collapsing between parent-child elements. Use the one that fits better with the rest of your styling:

• Set display to other than block.
• Set float to other than none.
• Remove the margin, and use instead padding. For example if you had margin-top: 10px, replace with padding-top: 10px;.
• Remove the margin, and use instead position (absolute or relative) with attributes top, bottom, etc. For example if you had margin-top: 10px, replace with position: relative; top: 10px;.
• Add a padding or a border in the side where the margins are collapsing, to the parent element. The border can be 1px and transparent.
• Set overflow to other than visible to the parent element.

I know this is an old issue, I've come across it many times. The problem is that all of the fixes here are hacks that would potentially have unintended consequences.

First off, there's an easy explanation for the root problem. Due to the way that margin collapsing works, if the first element inside a container has a top margin, that top margin is effectively applied to the parent container itself. You can test this on your own by doing the following:

<div>
    <h1>Test</h1>
</div>

In a debugger, open this up and hover the div. You'll notice that the div itself actually is placed where the top-margin of the H1 element stops. This behavior is intended by the browser.

So there's an easy fix to this without having to resort to strange hacks, as most of the posts here do (no insults intended, its just the truth) - simply go back to the original explanation - ...if the first element inside a container has a top margin... - Following that, you'd therefore need the first element in a container to NOT have a top margin. Okay, but how do you do that without adding elements that don't interfere semantically with your document?

Easy! Pseudo-elements! You could do this via a class or a pre-defined mixin. Add a :before pseudo-element:

CSS via a class:

.top-margin-fix::before {   
    content: ' ';
    display: block;
    width: 100%;
    height: .0000001em;
}

With this, following the above markup example you would modify your div as such:

<div class="top-margin-fix">
    <h1>Test</h1>
</div>

Why does this work?

The first element in a container, if it has no top-margin, sets the position of the start of the next element's top margin. By adding a :before pseudo-element, the browser actually adds a non-semantic (in other words, good for SEO) element into the parent container before your first element.

Q. Why the height: .0000001em?

A. A height is required for the browser to push the margin element down. This height is effectively zero, but it will still allow you to add padding to the parent container itself. Since it's effectively zero, it won't have an effect on the layout of the container, either. Beautiful.

Now you can add a class (or better, in SASS/LESS, a mixin!) to fix this problem instead of adding weird display styles that will cause unexpected consequences when you want to do other things with your elements, purposefully eliminating margins on elements and/or replacing it with padding, or strange position/float styles that really aren't intended to resolve this issue.

display: flex will work in this case. Give parent element display: flex; and then give margin as per your requirement to the h1 tag.

This happens because of margin collapse. When child element touches the boundary of parent element and any of them applied with margins then :

1. The margin which is largest will win (applied).

2. if any of them having margin then both will share the same.

Solutions

1. apply border to parent which makes parent and child separates.
2. apply padding to parent which makes parent and child separates.
3. apply overflow rather than visible on parent.
4. use before or after to create virtual element in parent div which can differ margin applied child and parent.
5. create one html element between margin applied child and parent can also separates them .