---
id: css-structural-pseudo-classes
title: "CSS Structural Pseudo-classes"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["structural pseudo-classes", ":first-child", ":lang", "nth-child", "position-based selectors", "tree-position selectors"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.87
created_at: 2026-06-23
updated_at: 2026-06-23
review_reason: ""
merge_history: []
tags: ["css", "web", "frontend", "w3schools", "pseudo-classes", "selectors"]
raw_sources: ["https://www.w3schools.com/css/css_pseudo_classes_structural.asp"]
applied_in: []
github_commit: ""
---

# [[CSS Structural Pseudo-classes]]

## 🎯 한 줄 통찰 (One-line insight)
Structural pseudo-classes select elements based on their position in the document tree, letting you target things like the first child or language-tagged elements without adding extra classes. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Definition** — Structural pseudo-classes select elements based on their position in the document tree. [S1]
- **`:first-child`** — Matches an element that is the first child of another element. [S1]
- **`:lang()`** — Used to select elements with a `lang` attribute set to the specified value. [S1]
- **Position is relative to the parent** — selection depends on where the element sits among its siblings, not on a class or id. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **First-child targeting** — `selector:first-child` styles only the element that appears first inside its parent. [S1]
- **Combining position with descent** — `p em:first-child` vs `p:first-child em` produce different results depending on which element the `:first-child` is attached to. [S1]
- **Language-aware styling** — `:lang(code)` applies styles (e.g. quotation marks) only to elements carrying a matching `lang` value. [S1]

## 📖 세부 내용 (Details)
**Introduction** [S1]
Structural pseudo-classes select elements based on their position in the document tree.

**The `:first-child` pseudo-class** [S1]
The `:first-child` pseudo-class matches a specified element that is the first child of another element.

Style all first child `<p>` elements:
```css
p:first-child {
  color: blue;
}
```

Style the first `<em>` element in all `<p>` elements:
```css
p em:first-child {
  color: blue;
}
```

Style all `<em>` elements in all first child `<p>` elements:
```css
p:first-child em {
  color: blue;
}
```

**The `:lang()` pseudo-class** [S1]
The `:lang()` pseudo-class is used to select elements with a `lang` attribute with the specified value.

Define quotation marks for `<q>` elements with `lang="no"` (Norwegian):
```html
<html>
<head>
<style>
q:lang(no) {
  quotes: "~" "~";
}
</style>
</head>
<body>

<p>Some text <q lang="no">A quote in a paragraph</q> Some text.</p>

</body>
</html>
```

**Other structural pseudo-classes** [S1]

| Pseudo-class | Description |
| --- | --- |
| `:last-child` | Matches the last child of a parent |
| `:nth-child(n)` | Matches the nth child of a parent |
| `:nth-last-child(n)` | Matches the nth child from the end |
| `:only-child` | Matches an element that is the only child |
| `:first-of-type` | Matches the first element of its type |
| `:last-of-type` | Matches the last element of its type |

For a complete list, the source directs readers to the CSS Pseudo-classes Reference. [S1]

## 🛠️ 적용 사례 (Applied in summary)
The page's own applied examples are the `:first-child` rules (blue paragraphs / emphasized text) and the Norwegian `:lang(no)` quotation-mark demo. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
First-child selector (language: CSS):
```css
p:first-child {
  color: blue;
}
```
Language selector (language: CSS):
```css
q:lang(no) {
  quotes: "~" "~";
}
```

## ⚖️ 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:** 0.87
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[CSS Tutorial]]
- **관련 개념:** [[CSS Pseudo-elements]], [[CSS Selectors]], [[CSS Combinators]]
- **참조 맥락:** Referenced when targeting elements by their tree position instead of adding extra classes.

## 📚 출처 (Sources)
- [S1] W3Schools — CSS Structural Pseudo-classes — https://www.w3schools.com/css/css_pseudo_classes_structural.asp

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