2nd/10_Wiki/Topic_CSS/CSS_Selectors.md

---
id: css-selectors
title: "CSS Selectors"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["CSS selector", "element selector", "id selector", "class selector", "universal selector", "simple selectors"]
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", "selectors"]
raw_sources: ["https://www.w3schools.com/css/css_selectors.asp"]
applied_in: []
github_commit: ""
---

# [[CSS Selectors]]

## 🎯 한 줄 통찰 (One-line insight)
CSS selectors are used to "find" (select) the HTML elements you want to style, ranging from simple selectors that match by name, id, or class to combinator, pseudo-class, pseudo-element, and attribute selectors. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Purpose** — a CSS selector selects the HTML element(s) you want to style. [S1]
- **Five categories** — Simple selectors (by name, id, class), Combinator selectors, Pseudo-class selectors, Pseudo-elements selectors, and Attribute selectors. [S1]
- **Element selector** — selects elements based on the element name (e.g. `p`). [S1]
- **id selector** — uses the `#` character plus the id of a specific element; an id is unique within a page. [S1]
- **class selector** — uses a `.` followed by a class name; multiple elements can share a class. [S1]
- **Naming rule** — an id name cannot start with a number, and a class name cannot start with a number. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **By type** — `element { ... }` styles all elements of that tag. [S1]
- **By id** — `#id { ... }` styles the one element with that id. [S1]
- **By class** — `.class { ... }` styles every element carrying that class. [S1]
- **Type + class scoping** — `element.class { ... }` styles only those elements of a type that also carry the class. [S1]
- **Multiple classes** — an element can reference more than one class at once via a space-separated `class` attribute. [S1]

## 📖 세부 내용 (Details)
A CSS selector is used to find (select) the HTML elements you want to style. CSS selectors divide into five categories: Simple selectors (select elements based on name, id, class), Combinator selectors, Pseudo-class selectors, Pseudo-elements selectors, and Attribute selectors. [S1]

**The CSS element selector** selects HTML elements based on the element name. [S1]
```css
p {
  text-align: center;
  color: red;
}
```

**The CSS id selector** uses the id attribute of an HTML element to select a specific element. The id of an element is unique within a page, so it selects one unique element. To select an element with a specific id, write a hash (`#`) character followed by the id of the element. [S1]
```css
#para1 {
  text-align: center;
  color: red;
}
```
Note: an id name cannot start with a number! [S1]

**The CSS class selector** selects HTML elements with a specific class attribute. To select elements with a specific class, write a period (`.`) character followed by the class name. [S1]
```css
.center {
  text-align: center;
  color: red;
}
```

You can also specify that only specific HTML elements should be affected by a class. In this example only `<p>` elements with `class="center"` will be center-aligned and red: [S1]
```css
p.center {
  text-align: center;
  color: red;
}
```

HTML elements can also refer to more than one class: [S1]
```html
<p class="center large">This paragraph refers to two classes.</p>
```
Note: a class name cannot start with a number! [S1]

**The CSS universal selector** (`*`) selects all HTML elements on the page. [S1]
```css
* {
  text-align: center;
  color: blue;
}
```

**The CSS grouping selector** selects all the HTML elements with the same style definitions. To group selectors, separate each selector with a comma. [S1]
```css
h1, h2, p {
  text-align: center;
  color: red;
}
```

**All CSS Simple Selectors** [S1]

| Selector | Example | Example description |
| --- | --- | --- |
| `#id` | `#firstname` | Selects the element with id="firstname" |
| `.class` | `.intro` | Selects all elements with class="intro" |
| `element.class` | `p.intro` | Selects only `<p>` elements with class="intro" |
| `*` | `*` | Selects all elements |
| `element` | `p` | Selects all `<p>` elements |
| `element,element,..` | `div, p` | Selects all `<div>` elements and all `<p>` elements |

## 🛠️ 적용 사례 (Applied in summary)
The page's own examples apply each selector type to style headings and paragraphs (element, id, class, type+class, universal, and grouping). No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Selecting by id and by class (language: CSS):
```css
#para1 {
  text-align: center;
  color: red;
}

.center {
  text-align: center;
  color: red;
}
```
Scoping a class to a specific element type:
```css
p.center {
  text-align: center;
  color: red;
}
```

## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
| Selector | When to use | Scope |
| --- | --- | --- |
| `element` (e.g. `p`) | Style every element of a tag uniformly | All elements of that type [S1] |
| `#id` (e.g. `#para1`) | Style one unique element | A single element (id is unique per page) [S1] |
| `.class` (e.g. `.center`) | Reuse a style across many elements | All elements carrying that class [S1] |
| `element.class` (e.g. `p.center`) | Reuse a class but limit it to one tag | Only that element type with the class [S1] |
| `*` | Apply a baseline to everything | All elements on the page [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:** 0.89
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[CSS Tutorial]]
- **관련 개념:** [[CSS Syntax]], [[CSS Grouping Selectors]], [[CSS Introduction]]
- **참조 맥락:** Referenced whenever deciding how to target HTML elements for styling.

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

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