html - Why is setting the top-margin of this child element pushing its parent container down with it?

Question

I have two divs:

<div id="headercontainer" data-type="background" data-speed="5"> 
    <div id="headersubcontainer">
        <h1>Simple and Cost Effective Web Solutions</h1>  
    </div> 
</div>

<div id="teamcontainer" data-type="background" data-speed="5"> 
    <div id="teamsubcontainer"> 
        <h1>Developed by a dedicated team</h1> 
    </div> 
</div> 

both have 100% widths and heights of 800px. The first heading I have set a top-margin: of 160px. Instead of moving the header lower into its parent div, it moves the parent div down with it as you can see in this picture:

Here is my relevant css:

h1{
    font-size: 48px;
    font-family: $header-font-stack; 
    font-weight: 100;
    width: 400px;
}

#headercontainer{
    width: 100%; 
    height: 800px;
    background-image: image-url("background.jpg");
    background-position: center top;
    background-repeat: no-repeat;
}

#headercontainer h1{
    text-align: center;
    margin: 0px auto;
    margin-top: 160px;
    color: #610B21;
}

Using a padding works obviously, but I would like to be more proper and use a margin. How can set a top margin and move the heading lower into the container without moving the container with it?

Answer 1

This is due to margin collapsing:

Top and bottom margins of blocks are sometimes combined (collapsed) into a single margin whose size is the largest of the margins combined into it, a behavior known as margin collapsing.

This is resulting in the parent element reverse-inheriting the child element top margin.

You can prevent this by adding   before the child element

Demo Fiddle

....or applying any of the below to the parent:

1. float: left / right
2. position: absolute
3. display: inline-block

Adding display:inline-block; to the parent likely being the preference if it is set to have a width to 100%

Demo Fiddle