How Do Nested Vertical Margin Collapses Work in CSS?

Understanding the Nuances of Nested Vertical Margin Collapses

In CSS, the concept of margin collapsing plays a crucial role in determining the spacing between elements on a web page. When margins of vertically adjacent elements touch, they collapse into a single margin, resulting in unexpected behavior. Nested vertical margin collapses further complicate this behavior.

Mechanism of Nested Margin Collapses

When two elements are nested within each other and their margins touch, the following two rules apply:

1. Margin Collapse: The margins collapse into a single margin with the maximum height of the individual margins.
2. Snuggling: The inner element "snuggles" against the top border of its container, eliminating any space between the elements.

For instance, consider the following HTML and CSS:

<code class="html"><div id="outer">
  <div id="inner">
    A
  </div>
</div></code>

<code class="css">#outer {
  margin-top: 10px;
  background: blue;
  height: 100px;
}
#inner {
  margin-top: 20px;
  background: red;
  height: 33%;
  width: 33%;
}</code>

In this case, the margins collapse to 20px (the maximum of 10px and 20px), and the inner div snuggles up to the top of the outer div.

However, if any content or border separates the two margins, the collapse is lost. This can occur when adding non-breaking whitespace or a border between the elements, as demonstrated below:

<code class="html"><div id="outer">
  <div id="inner">
     
    A
  </div>
</div></code>

Note: This behavior does not apply to elements positioned as absolute, fixed, or floated.

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