docs(10_Wiki): W3Schools 위키화 — HTML/CSS/JavaScript(core)

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>
2026-06-23 19:21:18 +09:00
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+---
+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).