---
id: css-z-index
title: "CSS Z-index"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["z-index", "stack order", "stacking order", "CSS z index", "layer order", "stacking context"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.88
created_at: 2026-06-23
updated_at: 2026-06-23
review_reason: ""
merge_history: []
tags: ["css", "web", "frontend", "w3schools", "position", "z-index"]
raw_sources: ["https://www.w3schools.com/css/css_z-index.asp"]
applied_in: []
github_commit: ""
---

# [[CSS Z-index]]

## 🎯 한 줄 통찰 (One-line insight)
The `z-index` property sets the stack order of positioned elements — higher values render in front of lower ones — and it only works on positioned elements and flex items. [S1]

## 🧠 핵심 개념 (Core concepts)
- **stack order** — `z-index` specifies the stack order of positioned elements, determining which elements appear in front of or behind others. [S1]
- **positive or negative** — elements can have positive or negative z-index values. [S1]
- **higher wins** — higher z-index values always appear above lower ones. [S1]
- **only on positioned elements** — z-index only works on positioned elements (`position: absolute`, `position: relative`, `position: fixed`, or `position: sticky`) and flex items. [S1]
- **default order** — without a z-index, elements render in HTML source order. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Send behind with negative z-index** — give an absolutely positioned element `z-index: -1` to render it behind text/content. [S1]
- **Explicit layering** — assign ascending z-index values to sibling boxes to control which overlaps which, independent of source order. [S1]

## 📖 세부 내용 (Details)
**CSS The z-index Property** [S1]
`z-index` specifies the stack order of positioned elements, determining which elements appear in front or behind others. Elements can have positive or negative z-index values.

**Note:** z-index only works on positioned elements (`position: absolute`, `position: relative`, `position: fixed`, or `position: sticky`) and flex items. [S1]

**First example — negative z-index puts an image behind text** [S1]
The CSS rule uses `position: absolute`, `left: 0px`, `top: 0px`, and `z-index: -1`. The HTML includes a heading "This is a heading". The image with z-index of -1 renders behind the text. The page did not provide the full verbatim CSS rule block — Not found in source.

**Second example — higher z-index appears above lower** [S1]
CSS classes defined:
- `.container` with `position: relative`
- `.black-box` with `z-index: 1`, plus border and margins
- `.gray-box` with `z-index: 3`, light gray background
- `.green-box` with `z-index: 2`, light green background

HTML structure: a container div holding three nested divs representing the three boxes. The gray box (z-index 3) sits on top, then green (2), then black (1). The full verbatim CSS/HTML block was not provided in the fetched source — Not found in source.

**Third example — rendering order without z-index** [S1]
Code identical to the second example but omitting all z-index declarations, showing that elements render in HTML source order.

**Property reference** [S1]

| Property | Description |
|----------|-------------|
| z-index | Sets the stack order of an element |

## 🛠️ 적용 사례 (Applied in summary)
The page's applied examples are: an image sent behind a heading via `z-index: -1`; three overlapping boxes layered by ascending z-index (gray 3 on top, green 2, black 1); and the same three boxes without z-index to show default source-order stacking. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Send a positioned element behind content (language: CSS):
```css
img {
  position: absolute;
  left: 0px;
  top: 0px;
  z-index: -1;
}
```

Layer sibling boxes by explicit stack order (language: CSS):
```css
.container {
  position: relative;
}
.black-box {
  z-index: 1;
}
.green-box {
  z-index: 2;
}
.gray-box {
  z-index: 3;
}
```
Note: the page described these declarations but did not present the complete rule blocks verbatim; the snippets above reflect the described properties — additional rule details Not found in source. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
No contradictions found in the source.

## ✅ 검증 상태 및 신뢰도
- **상태:** draft
- **검증 단계:** conceptual (실제 적용 사례 발견 시 applied/validated로 승격 가능)
- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
- **신뢰 점수:** 0.88
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[CSS Tutorial]]
- **관련 개념:** [[CSS Position Fixed and Absolute]], [[CSS Position Sticky]], [[CSS Position Offsets]]
- **참조 맥락:** Referenced whenever positioned elements overlap and their front-to-back order must be controlled.

## 📚 출처 (Sources)
- [S1] W3Schools — CSS Z-index — https://www.w3schools.com/css/css_z-index.asp

## 📝 변경 이력 (Change history)
- 2026-06-23: Initial draft synthesized from the W3Schools "CSS Z-index" page (Astra wiki-curation, P-Reinforce v3.1 format).