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W3Schools 튜토리얼을 P-Reinforce v3.1 포맷으로 위키화(영어 본문, 한/영 섹션 헤더). - Topic_HTML: 59문서 (튜토리얼+예제, 레퍼런스/메타 제외) - Topic_CSS: 190문서 (메인 + Advanced/Flexbox/Grid/RWD 전체) - Topic_JavaScript: 120문서 (코어 언어; Temporal/DOM상세/BOM/WebAPI/AJAX/jQuery/Graphics 등은 후속) 각 폴더 00_INDEX.md(MOC) 포함. 코드 verbatim, 미확인분은 "Not found in source" 표기. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
116 lines
5.2 KiB
Markdown
116 lines
5.2 KiB
Markdown
---
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id: css-z-index
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title: "CSS Z-index"
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category: "Frontend"
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status: "draft"
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verification_status: "conceptual"
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canonical_id: ""
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aliases: ["z-index", "stack order", "stacking order", "CSS z index", "layer order", "stacking context"]
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duplicate_of: ""
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source_trust_level: "B"
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confidence_score: 0.88
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created_at: 2026-06-23
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updated_at: 2026-06-23
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review_reason: ""
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merge_history: []
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tags: ["css", "web", "frontend", "w3schools", "position", "z-index"]
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raw_sources: ["https://www.w3schools.com/css/css_z-index.asp"]
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applied_in: []
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github_commit: ""
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---
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# [[CSS Z-index]]
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## 🎯 한 줄 통찰 (One-line insight)
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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]
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## 🧠 핵심 개념 (Core concepts)
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- **stack order** — `z-index` specifies the stack order of positioned elements, determining which elements appear in front of or behind others. [S1]
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- **positive or negative** — elements can have positive or negative z-index values. [S1]
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- **higher wins** — higher z-index values always appear above lower ones. [S1]
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- **only on positioned elements** — z-index only works on positioned elements (`position: absolute`, `position: relative`, `position: fixed`, or `position: sticky`) and flex items. [S1]
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- **default order** — without a z-index, elements render in HTML source order. [S1]
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## 🧩 추출된 패턴 (Extracted patterns)
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- **Send behind with negative z-index** — give an absolutely positioned element `z-index: -1` to render it behind text/content. [S1]
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- **Explicit layering** — assign ascending z-index values to sibling boxes to control which overlaps which, independent of source order. [S1]
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## 📖 세부 내용 (Details)
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**CSS The z-index Property** [S1]
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`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.
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**Note:** z-index only works on positioned elements (`position: absolute`, `position: relative`, `position: fixed`, or `position: sticky`) and flex items. [S1]
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**First example — negative z-index puts an image behind text** [S1]
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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.
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**Second example — higher z-index appears above lower** [S1]
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CSS classes defined:
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- `.container` with `position: relative`
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- `.black-box` with `z-index: 1`, plus border and margins
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- `.gray-box` with `z-index: 3`, light gray background
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- `.green-box` with `z-index: 2`, light green background
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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.
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**Third example — rendering order without z-index** [S1]
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Code identical to the second example but omitting all z-index declarations, showing that elements render in HTML source order.
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**Property reference** [S1]
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| Property | Description |
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|----------|-------------|
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| z-index | Sets the stack order of an element |
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## 🛠️ 적용 사례 (Applied in summary)
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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.
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## 💻 코드 패턴 (Code patterns)
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Send a positioned element behind content (language: CSS):
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```css
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img {
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position: absolute;
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left: 0px;
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top: 0px;
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z-index: -1;
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}
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```
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Layer sibling boxes by explicit stack order (language: CSS):
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```css
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.container {
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position: relative;
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}
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.black-box {
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z-index: 1;
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}
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.green-box {
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z-index: 2;
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}
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.gray-box {
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z-index: 3;
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}
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```
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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]
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## ⚖️ 모순 및 업데이트 (Contradictions & updates)
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No contradictions found in the source.
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## ✅ 검증 상태 및 신뢰도
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- **상태:** draft
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- **검증 단계:** conceptual (실제 적용 사례 발견 시 applied/validated로 승격 가능)
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- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
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- **신뢰 점수:** 0.88
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- **중복 검사 결과:** 신규 생성 (New discovery)
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## 🔗 지식 그래프 (Knowledge Graph)
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- **상위/루트:** [[CSS Tutorial]]
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- **관련 개념:** [[CSS Position Fixed and Absolute]], [[CSS Position Sticky]], [[CSS Position Offsets]]
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- **참조 맥락:** Referenced whenever positioned elements overlap and their front-to-back order must be controlled.
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## 📚 출처 (Sources)
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- [S1] W3Schools — CSS Z-index — https://www.w3schools.com/css/css_z-index.asp
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## 📝 변경 이력 (Change history)
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- 2026-06-23: Initial draft synthesized from the W3Schools "CSS Z-index" page (Astra wiki-curation, P-Reinforce v3.1 format).
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