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⬇️ CSS Margin Collapse — Simplified for GoNimbus

When working with vertical spacing in CSS, you may notice that margins don’t always add up the way you expect. This behavior is called margin collapse.

Margin collapse happens when the top margin of one element meets the bottom margin of another element, and instead of adding together, they merge into a single margin.

👉 Important: Margin collapse only occurs vertically (top and bottom), never on left and right margins.

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1️⃣ How Margin Collapse Works

Let’s see an example:

h1 {
  margin-bottom: 50px;
}

h2 {
  margin-top: 20px;
}

You might expect the space between <h1> and <h2> to be 70px (50 + 20).
But in reality, the collapsed margin = 50px (the larger value).

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2️⃣ Collapsing Between Sibling Elements

If multiple elements are stacked with margins, only the largest margin survives.

p {
  margin-top: 30px;
  margin-bottom: 30px;
}

Expected spacing: 60px (30 + 30)
Actual spacing: 30px (due to collapse).

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3️⃣ Collapsing Between Parent and Child

Margin collapse also happens when a child element’s margin touches its parent’s edge.

div {
  border: 1px solid black;
}

p {
  margin-top: 40px;
}

Here, the <div> has no padding or border. The child <p>’s top margin collapses outside the parent, creating 40px space outside the <div>.

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4️⃣ Preventing Margin Collapse

Sometimes you don’t want collapsing. You can prevent it using:

✔ Padding or Borders

div {
  padding-top: 1px;  /* prevents collapse */
}

✔ Overflow Property

div {
  overflow: auto;  /* keeps child margins inside */
}

✔ Flexbox or Grid Layouts
Modern layout systems like Flexbox and Grid do not collapse margins.

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5️⃣ Why Margin Collapse Exists

Margin collapse is a CSS optimization feature. It prevents stacked elements from creating double-spacing and keeps layouts consistent.

But for modern responsive design, many developers prefer padding or flexbox spacing utilities to avoid unexpected behavior.

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🧩 Exercise

Question:
If one <div> has margin-bottom: 40px; and the next <div> has margin-top: 60px;, what is the vertical space between them?

• 100px
• 20px
• 60px
• 0px

✔ Correct: The larger margin (60px) is used.

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📋 Quick Reference

Property	Description
margin	Shorthand for all margins
margin-top	Sets the top margin
margin-right	Sets the right margin
margin-bottom	Sets the bottom margin
margin-left	Sets the left margin
margin-collapse	Not a property — it’s default CSS behavior

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💡 GoNimbus Tip:
If you want precise control over spacing in modern projects, consider using:

• gap property in Flexbox and Grid (instead of margins)
• Padding for internal spacing
• Utility classes in frameworks (like Tailwind or Bootstrap) to avoid margin collapse confusion

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