---
id: wiki-2026-0508-render-tree
title: Render Tree
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Browser-Render-Tree, Layout-Tree]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [browser, rendering, performance, web]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: javascript
  framework: web-platform
---

# Render Tree

## 매 한 줄
> **"매 DOM × CSSOM = Render Tree"**. Browser는 매 parsed HTML (DOM) 과 매 styles (CSSOM) 을 매 merge — visible nodes만 매 골라 layout, paint, composite의 input 으로 사용. 2026 Chromium은 매 LayoutNG + RenderingNG로 매 진화.

## 매 핵심

### 매 Pipeline (Critical Rendering Path)
1. **HTML → DOM** (tokenize + tree construct).
2. **CSS → CSSOM** (parse stylesheets, compute cascade).
3. **DOM + CSSOM → Render Tree** — 매 `display: none` 노드는 매 제외, `::before`/`::after` 매 포함.
4. **Layout (Reflow)** — geometry 계산.
5. **Paint** — pixel commands → layers.
6. **Composite** — GPU layer merge → screen.

### 매 Render Tree ≠ DOM
- DOM 의 모든 노드 의 X. `<head>`, `<script>`, `display:none` 매 제외.
- `visibility: hidden` 의 매 포함됨 (매 space 차지).
- Pseudo-elements (`::before`) 매 추가됨 (DOM 에 없음).

### 매 Blocking
- **CSS 의 render-blocking by default**: CSSOM 완성 전에는 매 render tree 못 만듦.
- `<script>` 의 parser-blocking — `defer`/`async`로 완화.
- `<link rel="preload">` + `media` query 매 selective load.

### 매 2026 modern state (RenderingNG)
- **LayoutNG** — fragment tree로 매 incremental, isolation.
- **CompositeAfterPaint (CAP)** — paint ordering 의 매 composite 단계로 통합.
- **Off-main-thread scrolling/animation** — composite-only properties (`transform`, `opacity`) 매 main thread skip.
- **CSS Containment** (`contain: layout paint`) — subtree isolation.
- **content-visibility: auto** — 매 viewport 외 subtree skip layout/paint.

### 매 응용
1. Critical CSS inline → 매 first paint 단축.
2. `content-visibility: auto` 매 long page virtualization.
3. `will-change: transform` 매 layer promotion hint.

## 💻 패턴

### Critical CSS inline
```html
<head>
  <style>
    /* above-the-fold only */
    .hero { font-size: 3rem; color: #111; }
  </style>
  <link rel="preload" href="/main.css" as="style" onload="this.rel='stylesheet'">
</head>
```

### content-visibility virtualization
```css
article.long-section {
  content-visibility: auto;
  contain-intrinsic-size: 0 1000px;  /* placeholder size */
}
```

### Composite-only animation (avoid layout/paint)
```css
.menu { transform: translateX(-100%); transition: transform 200ms ease; }
.menu.open { transform: translateX(0); }
/* GPU composites; main thread free */
```

### Avoid layout thrashing
```javascript
// BAD: read-write-read-write triggers sync layout each iter
for (const el of items) {
  el.style.width = el.offsetWidth + 10 + 'px';
}

// GOOD: batch reads, then batch writes
const widths = items.map(el => el.offsetWidth);
items.forEach((el, i) => { el.style.width = widths[i] + 10 + 'px'; });
```

### CSS Containment
```css
.card {
  contain: layout style paint;
  /* descendant changes can't affect outside */
}
```

### Render-blocking analysis
```javascript
// Find render-blocking resources via PerformanceObserver
new PerformanceObserver((list) => {
  for (const entry of list.getEntries()) {
    if (entry.renderBlockingStatus === 'blocking') {
      console.warn('blocking:', entry.name);
    }
  }
}).observe({ type: 'resource', buffered: true });
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| First-paint 단축 | Critical CSS inline + async/defer JS |
| Long scrollable page | `content-visibility: auto` |
| Smooth animation | Composite-only (transform/opacity) + `will-change` |
| Widget isolation | `contain: layout paint` |
| Hidden tab UI | `display: none` (full skip) vs `visibility: hidden` (preserve space) |

**기본값**: composite-only animations, defer non-critical CSS/JS, contain on widgets.

## 🔗 Graph
- 부모: [[Browser-Rendering]] · [[Critical-Rendering-Path]]
- 응용: [[Web-Performance]] · [[Core Web Vitals Optimization (INP, LCP 개선)|Core-Web-Vitals]]
- Adjacent: [[Reflow_and_Repaint]] · [[CSSOM]] · [[DOM]]

## 🤖 LLM 활용
**언제**: performance audit explanations, CSS-perf debugging.
**언제 X**: production profiling은 매 real Chrome DevTools / Lighthouse / WebPageTest 사용 — LLM의 매 verification은 X.

## ❌ 안티패턴
- **Layout thrashing**: read/write 매 interleave → forced synchronous layout.
- **`top`/`left` animation**: triggers layout per frame; use `transform`.
- **거대 inline `<style>`**: matched HTML cache의 invalidation. Critical only.
- **`will-change` overuse**: 매 모든 element 에 적용 → memory blow-up.

## 🧪 검증 / 중복
- Verified (web.dev rendering performance, Chromium RenderingNG docs, MDN CRP).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — full rewrite covering DOM/CSSOM merge, blocking, modern RenderingNG |
