z-index — Complete Guide
z-index — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of CSS Tutorial on Toolliyo Academy.
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CSS Tutorial · Lesson 16 of 100
z-index
Foundations & Layout → Responsive & Motion → Architecture & Perf → Projects
Foundations & Layout · 1 — Style · ~12 min read · CSS — Layout Systems
1. Introduction
Today: z-index. Read the diagram, paste the CSS, then change one value and watch the UI update.
z-index stacks positioned elements on the z-axis. Higher values paint on top. Stacking context is created by position + z-index, opacity, transform, and more.
2. Real-world story
User confirms transfer; modal must block clicks on dashboard beneath.
Outcome: Documented z-index layers prevent overlay bugs in QA.
3. Why it matters
StyleVerse modals must sit above dropdowns, which sit above page content. Wrong z-index causes clicks to hit the wrong layer.
4. Visual understanding
Read this diagram — the mental model for z-index.
static → normal flow relative → nudge without leaving flow absolute → positioned vs nearest positioned ancestor fixed → vs viewport sticky → scrolls then sticks z-index stacks positioned layers
5. Key concepts (easy words)
| Idea | Meaning |
|---|---|
| What | z-index stacks positioned elements on the z-axis. Higher values paint on top. Stacking context is created by position + z-index, opacity, transform, and more. |
| Remember | z-index orders overlapping positioned layers. Stacking contexts isolate child z-index values. Use a token scale for modals, dropdowns, tooltips. |
| Practice tip | Change one property, refresh, watch DevTools Computed. |
6. How it works
- Overlay at 1000 dims background. Modal at 1001 centers above overlay for confirm dialogs.
- z-index orders overlapping positioned layers.
- Stacking contexts isolate child z-index values.
- Check the result in DevTools → Computed and the box model overlay.
7. Choose wisely
| Option | Notes |
|---|---|
| Do | Practice z-index on a small StyleVerse card |
| Avoid | Copying huge frameworks before learning core CSS |
8. Try this example
Paste into a <style> block (or use Run Example). Refresh to see StyleVerse UI changes.
.overlay { position: fixed; inset: 0; background: rgba(0,0,0,0.5); z-index: 1000; }
.modal { position: fixed; top: 50%; left: 50%; transform: translate(-50%,-50%); z-index: 1001; background: #fff; padding: 24px; border-radius: 8px; }Line walkthrough
| Code | What it means |
|---|---|
.overlay { position: fixed; inset: 0; background: rgba(0,0,0,0.5); z-index: 1000; } | Selector — picks which elements get these declarations. |
.modal { position: fixed; top: 50%; left: 50%; transform: translate(-50%,-50%); z-index: 1001; backg | Selector — picks which elements get these declarations. |
Run Example »
Edit the code below and click Run to see the result in Toolliyo’s live editor.
9. Another real-world angle
10. Best practices checklist
- Prefer classes over ids for reusable StyleVerse components.
- Use design tokens (CSS variables) for color and spacing.
- Mobile-first media queries; test keyboard focus.
- Keep specificity low — avoid !important except rare overrides.
- Animate transform/opacity when you can; respect prefers-reduced-motion.
11. Common mistakes
- z-index on static elements — has no effect without position.
- Random z-index values like 99999 — use a documented scale.
12. Practice in the browser
- Open modal with overlay behind it.
- Lower modal z-index below overlay — modal disappears under dimmer.
- Inspect stacking context in DevTools Layers panel.
- Define --sv-z-modal: 1001 token for consistency.
Experiments
- Add .toast { z-index: 1100; } above modals.
- Set dropdown z-index: 500 below modal layer.
13. FAQ
Where should I put CSS for z-index?
Start in a <style> block or styles.css linked from HTML. Later use CSS Modules, Tailwind, or tokens in a design system.
Why is my rule not applying?
Check selector match, typos, specificity, and whether a more specific rule or inline style wins. DevTools Computed tells the truth.
Flexbox or Grid for this?
Flex for one direction (toolbars, card rows). Grid for two-dimensional page frames. Many UIs use both.
14. Interview questions
Explain z-index simply.
z-index stacks positioned elements on the z-axis. Higher values paint on top. Stacking context is created by position + z-index, opacity, transform, and more.
How do you debug CSS?
Inspect element, toggle declarations, read Computed styles, outline the box model, and reduce specificity conflicts.
How does this show up in production UI?
Documented z-index layers prevent overlay bugs in QA.
15. Remember
- z-index orders overlapping positioned layers.
- Stacking contexts isolate child z-index values.
- Use a token scale for modals, dropdowns, tooltips.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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