Understanding Nested Vertical Margin Collapse for CSS Newbies
Nested vertical margin collapse is a fundamental concept in CSS layout that governs how margins interact when elements are nested within each other. To comprehend it, let's explore the rules that determine this behavior:
- Margin Collapse: When two margins meet, they collapse into a single margin, with the collapsed value being the maximum of the two individual margins.
- Margin Snuggling: If only margin separates a nested element from its containing element, the nested element will "snuggle" to the start of the containing element.
- Flow Exception: Margin collapse and snuggling do not apply to elements outside the normal flow, such as floated, positioned absolutely, or positioned fixed elements.
Consider the following example to illustrate these rules:
<code class="html"><div id="outer">
<div id="inner">
A
</div>
</div></code>
Given these styles:
<code class="css">#outer {
margin-top: 10px;
background: blue;
height: 100px;
}
#inner {
margin-top: 20px;
background: red;
height: 33%;
width: 33%;
}</code>
- The outer and inner margins collapse because they touch, resulting in a combined margin of 20px.
- The inner div snuggles to the top of the outer div since nothing separates them.
However, the slightest change, such as adding a non-breaking space character between the elements or giving the inner div a border, will prevent the margins from collapsing. This is because the space or border creates a separation between the margins.
Understanding these rules enables developers to predict and control the layout behavior of nested elements, ensuring cross-browser consistency and predictable results.
The above is the detailed content of How Does Nested Vertical Margin Collapse Work in CSS?. For more information, please follow other related articles on the PHP Chinese website!
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