How Does Vertical Margin Collapse Work With Nested Elements in CSS?

Understanding Nested Vertical Margin Collapse

The concept of vertical margin collapse in CSS can be perplexing, especially when nesting elements. Let's break down the mechanics with a simplified explanation.

Two essential rules govern margin collapse:

1. Touching Margins Collapse: When the vertical margins of adjacent elements touch, they collapse into a single margin.
2. Nested Snuggle: If only margin separates nested elements, they "snuggle" together, eliminating any space created by the inner element's margin.

Consider the following HTML structure:

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

And the initial CSS styles:

<code class="CSS">#outer {
    margin-top: 10px;
    background: blue;
    height: 100px;
}

#inner {
    margin-top: 20px;
    background: red;
    height: 33%;
    width: 33%;
}</code>

In this scenario, the nested div's margin (20px) triggers a margin collapse with the outer div's margin (10px), resulting in a total margin of 20px for the entire block. The inner div then snuggles up to the start of the container due to the lack of non-margin separation.

However, if we introduce any separation between the margins, such as a border or even a non-breaking space, the collapse is broken. This is because margins that are no longer considered "touching" do not collapse.

Remember, these rules do not apply to elements outside the regular flow (e.g., floated or absolutely positioned elements). Grasping these concepts will help you develop a clear understanding of how vertical margins behave when nesting elements in CSS.

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