What Is the Default Stacking Order of Elements Without z-index?

Before diving into complex UI layering with z-index, it’s important to understand how browsers stack HTML elements by default — even when no z-index is applied.

So, what is the default stacking order of elements without z-index?

Let’s break it down with clear explanations and practical examples.

Table of Contents

📚 What Is Stacking Order?

In web design, the stacking order refers to the order in which elements are rendered on the z-axis (front-to-back). Think of it like sheets of paper stacked on top of each other — some will appear in front, others behind.

🔤 Default Stacking Order: No `z-index` Needed

Here’s the browser’s default stacking order, from back to front, when no z-index is defined:

Root element (<html>)
Non-positioned block-level elements (natural document flow)
Floats
Inline elements
Positioned elements with z-index: auto
Positioned elements with z-index values (when present)

So, without z-index, the order is primarily determined by HTML source order and positioning.

📌 Example: Default Order Without `z-index`

<div class="box1">Box 1</div>
<div class="box2">Box 2</div>

.box1, .box2 {
  position: relative; /* no z-index set */
  margin-top: -40px;  /* creates overlap */
}

✅ Result: Box 2 appears on top of Box 1 because it comes later in the DOM.

🎯 Key Rules of Default Stacking

1. HTML Source Order Matters

If two elements overlap and have no z-index, the one that comes later in the DOM appears on top.

2. Positioning Creates a Stacking Context

position: relative, absolute, fixed, or sticky can change how elements stack, even without z-index.

3. Floats Stack Above Non-Floats

If you float an element, it naturally appears above non-floated siblings.

📊 Stacking Order Diagram (Without `z-index`)

⬆ Top of screen (front)

1. Positioned elements (no z-index)
2. Inline elements
3. Floated elements
4. Block elements (in DOM order)
5. Root element

⬇ Bottom of screen (back)

🧠 What Happens When You Add `z-index`?

As soon as you add a z-index, the stacking context changes. Higher z-index values take precedence within the same stacking context.

.box1 {
  position: relative;
  z-index: 10;
}

✅ Now box1 will appear on top, regardless of DOM order.

🛠 Tips for Managing Stacking Order

Tip	Why It Helps
Be aware of HTML source order	Default stacking depends on it
Use `position` + `z-index` wisely	Creates predictable layering
Avoid magic numbers	Stick to a layering scale (e.g., 10, 100, 1000)
Understand stacking context	Some parents can trap child elements beneath others

✅ Summary

Concept	Description
Default stacking order	Determined by DOM order and element type
Positioning required?	No, but `position` can affect stacking
Floats vs. blocks	Floats appear above normal blocks
Inline vs. block	Inline elements stack above blocks
Later in DOM = on top	True when `z-index` is not used

🚀 Final Thoughts

Understanding the default stacking order is essential before reaching for z-index. Often, your layout issue can be solved by reordering elements or applying thoughtful positioning — not just cranking up z-index values.