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10_Wiki/Topic_Programming/Topic_CSS/CSS_Margin_Collapse.md

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+---
+id: css-margin-collapse
+title: "CSS Margin Collapse"
+category: "Frontend"
+status: "draft"
+verification_status: "conceptual"
+canonical_id: ""
+aliases: ["margin collapse", "collapsing margins", "CSS margin collapsing", "vertical margin collapse", "largest margin wins"]
+duplicate_of: ""
+source_trust_level: "B"
+confidence_score: 0.89
+created_at: 2026-06-23
+updated_at: 2026-06-23
+review_reason: ""
+merge_history: []
+tags: ["css", "web", "frontend", "w3schools", "margin", "box-model"]
+raw_sources: ["https://www.w3schools.com/css/css_margin_collapse.asp"]
+applied_in: []
+github_commit: ""
+---
+
+# [[CSS Margin Collapse]]
+
+## 🎯 한 줄 통찰 (One-line insight)
+Top and bottom margins of elements are sometimes collapsed into a single margin that is equal to the largest of the two margins — this only happens with top and bottom margins, never left and right. [S1]
+
+## 🧠 핵심 개념 (Core concepts)
+- **Definition** — top and bottom margins of elements are sometimes collapsed into a single margin that is equal to the largest of the two margins. [S1]
+- **Largest wins** — the resulting single margin equals the larger of the two adjacent margins, not their sum. [S1]
+- **Vertical only** — this does not happen on left and right margins, only top and bottom margins. [S1]
+
+## 🧩 추출된 패턴 (Extracted patterns)
+- **max, not sum** — when two vertical margins meet, the gap is `max(margin_a, margin_b)`, not `margin_a + margin_b`. [S1]
+- **Horizontal exemption** — never rely on collapse for left/right spacing; it applies only to top/bottom. [S1]
+
+## 📖 세부 내용 (Details)
+**CSS Margin Collapse** [S1]
+Top and bottom margins of elements are sometimes collapsed into a single margin that is equal to the largest of the two margins.
+
+This does not happen on left and right margins! Only top and bottom margins! [S1]
+
+**Example** — an `h1` with a bottom margin and an `h2` with a top margin: [S1]
+```css
+h1 {
+  margin: 0 0 50px 0;
+}
+
+h2 {
+  margin: 20px 0 0 0;
+}
+```
+The `h1` element has a bottom margin of 50px and the `h2` element has a top margin set to 20px. Common sense would seem to suggest that the vertical margin between the `h1` and the `h2` would be a total of 70px (50px + 20px). But due to margin collapse, the actual margin ends up being 50px. [S1]
+
+## 🛠️ 적용 사례 (Applied in summary)
+The page's own applied example is the `h1`/`h2` stylesheet above, which demonstrates that adjacent 50px and 20px vertical margins collapse to a single 50px gap rather than summing to 70px. No external project/commit applications found in the source.
+
+## 💻 코드 패턴 (Code patterns)
+Adjacent vertical margins that collapse to the larger value (language: CSS):
+```css
+h1 {
+  margin: 0 0 50px 0;
+}
+
+h2 {
+  margin: 20px 0 0 0;
+}
+```
+
+## ⚖️ 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:** 0.89
+- **중복 검사 결과:** 신규 생성 (New discovery)
+
+## 🔗 지식 그래프 (Knowledge Graph)
+- **상위/루트:** [[CSS Tutorial]]
+- **관련 개념:** [[CSS Margins]], [[CSS Border Sides]], [[CSS Border Shorthand]]
+- **참조 맥락:** A box-model gotcha to keep in mind whenever vertical spacing between stacked elements matters.
+
+## 📚 출처 (Sources)
+- [S1] W3Schools — CSS Margin Collapse — https://www.w3schools.com/css/css_margin_collapse.asp
+
+## 📝 변경 이력 (Change history)
+- 2026-06-23: Initial draft synthesized from the W3Schools "CSS Margin Collapse" page (Astra wiki-curation, P-Reinforce v3.1 format).