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koriweb 9609c04755 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

4.9 KiB

Raw Blame History

id, title, category, status, verification_status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, created_at, updated_at, review_reason, merge_history, tags, raw_sources, applied_in, github_commit

title

CSS Opacity

🎯 한 줄 통찰 (One-line insight)

The opacity property sets how transparent an element is on a 0.0–1.0 scale, but because it affects the whole element (including children), rgba() is preferred when only the background should be see-through. [S1]

🧠 핵심 개념 (Core concepts)

opacity property — specifies the opacity/transparency of an element. [S1]
Value range — 0.0 is completely transparent, 1.0 is fully opaque. [S1]
Hover interaction — opacity can change on :hover to fade images in or out. [S1]
RGBA alternative — rgba() adds a fourth alpha value (0.0–1.0) to a color, making only that color transparent. [S1]

🧩 추출된 패턴 (Extracted patterns)

Fade-on-hover — pair a base opacity with an :hover opacity to brighten or dim images on pointer-over. [S1]
Transparent background only — use rgba() on background-color so the text/children stay fully opaque while the background is see-through. [S1]
Text-in-transparent-box — layer a transbox with an rgba() background over a patterned background div. [S1]

📖 세부 내용 (Details)

Opacity property [S1] The opacity property specifies the opacity/transparency of an element. It can take a value from 0.0 to 1.0, where 0.0 is completely transparent and 1.0 is fully opaque.

Basic image opacity:

img {
  opacity: 0.5;
}

Transparent image that becomes opaque on hover:

img {
  opacity: 0.5;
}

img:hover {
  opacity: 1.0;
}

Reversed hover effect (opaque image fades on hover):

img:hover {
  opacity: 0.5;
}

Transparent box:

div {
  opacity: 0.3;
}

Transparency using RGBA [S1] RGBA color values add a fourth alpha channel that specifies the opacity of the color, so only the background becomes transparent:

div {
  background: rgba(4, 170, 109, 0.3) /* Green background with 30% opacity */
}

Text in a transparent box [S1]

<html>
<head>
<style>
div.background {
  background: url(klematis.jpg) repeat;
  border: 2px solid black;
}

div.transbox {
  margin: 30px;
  background-color: rgba(255, 255, 255, 0.6);
  border: 1px solid black;
}

div.transbox p {
  margin: 5%;
  font-weight: bold;
  color: #000000;
}
</style>
</head>
<body>

<div class="background">
  <div class="transbox">
    <p>This is some text that is placed in the transparent box.</p>
  </div>
</div>

</body>
</html>

🛠️ 적용 사례 (Applied in summary)

The page's applied examples are the half-opacity image, hover fade-in/fade-out images, the 30%-opacity div, the RGBA green background, and the text-in-transparent-box layout. No external project/commit applications found in the source.

💻 코드 패턴 (Code patterns)

Fade image in on hover (language: CSS):

img {
  opacity: 0.5;
}

img:hover {
  opacity: 1.0;
}

Transparent background only, via RGBA (language: CSS):

div {
  background: rgba(255, 255, 255, 0.6);
}

⚖️ 비교 및 선택 기준 (Comparison & decision criteria)

opacity vs rgba() — The source distinguishes the two: opacity applies to the whole element and is shown fading entire images/boxes, whereas the "Transparency using RGBA" approach applies the alpha only to a color so a background can be transparent without dimming its content. Use opacity to fade an element as a whole; use rgba() when only the background color should be transparent (as in the text-in-transparent-box example). [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 Colors, CSS Backgrounds, CSS Images
참조 맥락: Referenced when fading elements or making backgrounds see-through.

📚 출처 (Sources)

[S1] W3Schools — CSS Opacity — https://www.w3schools.com/css/css_image_transparency.asp

📝 변경 이력 (Change history)

2026-06-23: Initial draft synthesized from the W3Schools "CSS Opacity" page (Astra wiki-curation, P-Reinforce v3.1 format).

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