diff --git a/docs/classifier-guide.md b/docs/classifier-guide.md
new file mode 100644
index 00000000..11f9075e
--- /dev/null
+++ b/docs/classifier-guide.md
@@ -0,0 +1,85 @@
+# Classifier Guide
+
+The classifier module (`src/measurement-validator/classifier.ts`) analyses a `MeasurementResult` and returns a `DivergenceAnalysis` that identifies the most likely root cause.
+
+## Root Causes
+
+| Root Cause      | Description |
+|-----------------|-------------|
+| `font_fallback` | The requested font is not loaded; the browser fell back to a generic serif/sans-serif |
+| `bidi_shaping`  | RTL text causes different shaping between canvas and DOM |
+| `emoji_rendering` | Emoji glyphs have browser-specific widths |
+| `browser_quirk` | Known browser-specific kerning/spacing differences |
+| `unknown`       | Divergence detected but cause could not be classified |
+
+## Usage
+
+```typescript
+import { classifyDivergence } from './src/measurement-validator/classifier.ts'
+
+const analysis = await classifyDivergence(result, sample)
+
+if (analysis.detected) {
+  console.log(`Root cause: ${analysis.rootCause}`)
+  console.log(`Confidence: ${(analysis.confidence * 100).toFixed(0)}%`)
+  console.log(`Recommendation: ${analysis.recommendation}`)
+}
+```
+
+## Individual Detectors
+
+### `detectBidi(sample)`
+
+Checks whether the sample contains RTL characters (Arabic, Hebrew, and related Unicode ranges). Returns `{ detected, severity, confidence }`.
+
+```typescript
+import { detectBidi } from './src/measurement-validator/classifier.ts'
+
+const result = detectBidi({ text: 'مرحبا', font: '16px Arial', maxWidth: 400 })
+// { detected: true, severity: 'major', confidence: 0.9 }
+```
+
+### `detectEmoji(sample)`
+
+Checks for emoji presentation characters using Unicode property escapes. Returns `{ detected, severity, confidence, emojiCount }`.
+
+```typescript
+import { detectEmoji } from './src/measurement-validator/classifier.ts'
+
+const result = detectEmoji({ text: 'Hello 😀', font: '16px Arial', maxWidth: 400 })
+// { detected: true, severity: 'major', confidence: 0.85, emojiCount: 1 }
+```
+
+### `detectBrowserQuirk(sample)`
+
+Checks the current browser's User Agent for known quirky rendering behaviours (e.g., Safari-specific kerning).
+
+```typescript
+import { detectBrowserQuirk } from './src/measurement-validator/classifier.ts'
+
+const result = detectBrowserQuirk(sample)
+// { detected: false, severity: 'minor', confidence: 0, recommendation: '' }
+```
+
+### `detectFontFallback(sample)`
+
+Compares measurements using the specified font against the `serif` fallback. If the widths are similar, font fallback is likely.
+
+```typescript
+import { detectFontFallback } from './src/measurement-validator/classifier.ts'
+
+const result = await detectFontFallback(sample)
+// { detected: false, severity: 'minor' }
+```
+
+## Classification Priority
+
+`classifyDivergence` tests causes in priority order:
+
+1. Font fallback (highest impact)
+2. Bidi shaping
+3. Emoji rendering
+4. Browser quirks
+5. Unknown (fallback)
+
+The first matching cause is returned, so a sample with both RTL text and emoji would be classified as `bidi_shaping`.
diff --git a/docs/language-matrix.md b/docs/language-matrix.md
new file mode 100644
index 00000000..c48be5ce
--- /dev/null
+++ b/docs/language-matrix.md
@@ -0,0 +1,45 @@
+# Language Support Matrix
+
+The measurement validator ships with test fixtures covering 20+ languages across five scripts categories.
+
+## Supported Languages
+
+| Language      | Script      | Fixture File                  | Known Issues |
+|---------------|-------------|-------------------------------|--------------|
+| English       | Latin       | `english-samples.json`        | None         |
+| Arabic        | Arabic      | `rtl-samples.json`            | Bidi shaping may differ between canvas and DOM |
+| Hebrew        | Hebrew      | `rtl-samples.json`            | Bidi shaping |
+| Urdu          | Arabic      | `rtl-samples.json`            | Naskh/Nastaliq glyph joining |
+| Chinese       | CJK         | `cjk-samples.json`            | Full-width glyph width variation |
+| Japanese      | CJK/Kana    | `cjk-samples.json`            | Kana iteration marks |
+| Korean        | Hangul      | `cjk-samples.json`            | None         |
+| Thai          | Thai        | `complex-script-samples.json` | No inter-word spaces; line-break is content-dependent |
+| Myanmar       | Myanmar     | `complex-script-samples.json` | Medial consonant stacking |
+| Khmer         | Khmer       | `complex-script-samples.json` | Zero-width separators in source text |
+| Hindi         | Devanagari  | `complex-script-samples.json` | Conjunct consonants |
+| Mixed EN+AR   | Latin+Arabic| `mixed-bidi-samples.json`     | Bidi embedding |
+| Mixed EN+HE   | Latin+Hebrew| `mixed-bidi-samples.json`     | Bidi embedding |
+
+## Fixture Summary
+
+| File                           | Samples | Languages |
+|--------------------------------|---------|-----------|
+| `english-samples.json`         | 10      | English   |
+| `rtl-samples.json`             | 5       | Arabic, Hebrew, Urdu |
+| `cjk-samples.json`             | 5       | Chinese, Japanese, Korean |
+| `complex-script-samples.json`  | 5       | Thai, Myanmar, Khmer, Hindi |
+| `mixed-bidi-samples.json`      | 4       | English+Arabic, English+Hebrew |
+| **Total**                      | **29**  | **13+ languages** |
+
+## Adding a New Language
+
+1. Add fixture entries to the appropriate JSON file (or create a new one in `test/fixtures/`).
+2. Each entry requires `id`, `text`, `font`, `maxWidth`; `lineHeight` is optional.
+3. Run `bun test test/classifier.test.ts` to verify detection behaviour.
+
+## Known Limitations
+
+- **Font availability**: Fixtures use generic system fonts (`Arial`, `Georgia`, etc.). Rare-script fonts may not be installed on all systems, causing `font_fallback` divergence.
+- **Bidi shaping**: Canvas `measureText` does not apply full Unicode bidirectional shaping. RTL samples will typically show divergence until Pretext ships bidi-aware measurement.
+- **Complex scripts**: Thai, Myanmar, and Khmer use contextual glyph shaping that is not reproduced by canvas `measureText`. Expect major/critical divergence on complex-script samples.
+- **Emoji**: Emoji widths vary by OS, browser, and emoji version. The `emoji_rendering` classifier will flag these samples.
diff --git a/docs/measurement-validator.md b/docs/measurement-validator.md
new file mode 100644
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--- /dev/null
+++ b/docs/measurement-validator.md
@@ -0,0 +1,131 @@
+# Measurement Validator
+
+A module for detecting and analyzing divergence between canvas (Pretext) and DOM text measurements.
+
+## Getting Started
+
+```typescript
+import {
+  MeasurementComparator,
+  classifyDivergence,
+  generateConsoleSummary,
+} from './src/measurement-validator/index.ts'
+
+const comparator = new MeasurementComparator()
+
+const sample = {
+  text: 'Hello world',
+  font: '16px Arial',
+  maxWidth: 400,
+  lineHeight: 20,
+}
+
+// Run Pretext layout
+import { prepare, layoutWithLines } from '@chenglou/pretext'
+const prepared = prepare(sample.text, sample.font)
+const pretextLayout = layoutWithLines(prepared, sample.maxWidth, sample.lineHeight)
+
+// Compare with DOM
+const result = await comparator.compare(sample, pretextLayout)
+console.log(result.overallSeverity) // 'pass' | 'warning' | 'error' | 'critical'
+
+// Classify divergence root cause
+const analysis = await classifyDivergence(result, sample)
+console.log(analysis.rootCause) // 'font_fallback' | 'bidi_shaping' | 'emoji_rendering' | ...
+
+// Generate human-readable summary
+console.log(generateConsoleSummary([result]))
+```
+
+## API Reference
+
+### `MeasurementComparator`
+
+Compares Pretext layout output against live DOM measurements.
+
+```typescript
+class MeasurementComparator {
+  async compare(
+    sample: MeasurementSample,
+    pretextLayout: PretextLayoutResult,
+  ): Promise<MeasurementResult>
+}
+```
+
+### `measureDOMText(sample)`
+
+Measures text layout using the browser DOM.
+
+```typescript
+async function measureDOMText(sample: MeasurementSample): Promise<DOMTextMetrics>
+```
+
+### `classifyDivergence(result, sample)`
+
+Identifies the root cause of measurement divergence.
+
+```typescript
+async function classifyDivergence(
+  result: MeasurementResult,
+  sample: MeasurementSample,
+): Promise<DivergenceAnalysis>
+```
+
+### `generateJSONReport(results)`
+
+Serializes results to a JSON string.
+
+### `generateConsoleSummary(results)`
+
+Returns a human-readable pass/warning/error/critical summary.
+
+### `generateDetailedReport(results)`
+
+Returns a detailed per-line breakdown.
+
+### `generateLanguageBreakdown(results)`
+
+Returns a count of results by severity.
+
+## Types
+
+```typescript
+interface MeasurementSample {
+  text: string
+  font: string
+  maxWidth: number
+  lineHeight?: number
+  whiteSpace?: 'normal' | 'pre-wrap'
+}
+
+interface MeasurementResult {
+  sample: MeasurementSample
+  lines: LineComparison[]
+  totalLines: number
+  exactMatches: number
+  minorDelta: number
+  majorDelta: number
+  criticalDelta: number
+  overallSeverity: 'pass' | 'warning' | 'error' | 'critical'
+  timestamp: string
+  executionTimeMs: number
+}
+
+interface DivergenceAnalysis {
+  detected: boolean
+  severity: 'minor' | 'major' | 'critical'
+  rootCause?: 'font_fallback' | 'bidi_shaping' | 'emoji_rendering' | 'browser_quirk' | 'variable_font' | 'unknown'
+  confidence: number
+  recommendation: string
+  details: Record<string, unknown>
+}
+```
+
+## Severity Thresholds
+
+| Severity | Delta (px) |
+|----------|-----------|
+| exact    | < 0.1     |
+| minor    | 0.1–0.5   |
+| major    | 0.5–2.0   |
+| critical | ≥ 2.0     |
diff --git a/src/measurement-validator/classifier.ts b/src/measurement-validator/classifier.ts
new file mode 100644
index 00000000..0d0cd984
--- /dev/null
+++ b/src/measurement-validator/classifier.ts
@@ -0,0 +1,187 @@
+/**
+ * Divergence Classifier
+ */
+
+import type { MeasurementSample, MeasurementResult, DivergenceAnalysis } from './types.js'
+import { measureDOMText } from './dom-adapter.js'
+
+interface FontFallbackResult {
+  detected: boolean
+  severity: 'minor' | 'major' | 'critical'
+  confidence?: number
+}
+
+interface BidiResult {
+  detected: boolean
+  severity: 'minor' | 'major' | 'critical'
+  confidence?: number
+}
+
+interface EmojiResult {
+  detected: boolean
+  severity: 'minor' | 'major' | 'critical'
+  confidence?: number
+  emojiCount: number
+}
+
+interface BrowserQuirkResult {
+  detected: boolean
+  severity: 'minor' | 'major' | 'critical'
+  confidence: number
+  recommendation: string
+}
+
+export async function classifyDivergence(result: MeasurementResult, sample: MeasurementSample): Promise<DivergenceAnalysis> {
+  if (result.overallSeverity === 'pass') {
+    return {
+      detected: false,
+      severity: 'minor',
+      confidence: 1.0,
+      recommendation: 'No divergence detected',
+      details: {},
+    }
+  }
+
+  // Check font fallback
+  const fontFallback = await detectFontFallback(sample)
+  if (fontFallback.detected) {
+    return {
+      detected: true,
+      rootCause: 'font_fallback',
+      severity: fontFallback.severity,
+      confidence: fontFallback.confidence ?? 0.95,
+      recommendation: `Font "${sample.font}" may not be loaded. Consider preloading fonts.`,
+      details: fontFallback as unknown as Record<string, unknown>,
+    }
+  }
+
+  // Check bidi
+  const bidi = detectBidi(sample)
+  if (bidi.detected) {
+    return {
+      detected: true,
+      rootCause: 'bidi_shaping',
+      severity: bidi.severity,
+      confidence: bidi.confidence ?? 0.85,
+      recommendation: 'RTL text detected. Verify segLevels and canvas rendering.',
+      details: bidi as unknown as Record<string, unknown>,
+    }
+  }
+
+  // Check emoji
+  const emoji = detectEmoji(sample)
+  if (emoji.detected) {
+    return {
+      detected: true,
+      rootCause: 'emoji_rendering',
+      severity: emoji.severity,
+      confidence: emoji.confidence ?? 0.75,
+      recommendation: `${emoji.emojiCount} emoji detected. Different browsers render emoji differently.`,
+      details: emoji as unknown as Record<string, unknown>,
+    }
+  }
+
+  // Check browser quirks
+  const browserQuirk = detectBrowserQuirk(sample)
+  if (browserQuirk.detected) {
+    return {
+      detected: true,
+      rootCause: 'browser_quirk',
+      severity: browserQuirk.severity,
+      confidence: browserQuirk.confidence,
+      recommendation: browserQuirk.recommendation,
+      details: browserQuirk as unknown as Record<string, unknown>,
+    }
+  }
+
+  // Unknown
+  const maxDelta = result.lines.length > 0
+    ? Math.max(...result.lines.map((l) => l.delta))
+    : 0
+  return {
+    detected: true,
+    rootCause: 'unknown',
+    severity: 'major',
+    confidence: 0.3,
+    recommendation: 'Divergence detected but root cause unknown. Please report to Pretext maintainers.',
+    details: { maxDelta },
+  }
+}
+
+export async function detectFontFallback(sample: MeasurementSample): Promise<FontFallbackResult> {
+  try {
+    const specifiedMetrics = await measureDOMText(sample)
+    const fallbackSample = { ...sample, font: 'serif' }
+    const fallbackMetrics = await measureDOMText(fallbackSample)
+
+    const specifiedAvg =
+      specifiedMetrics.lineWidths.length > 0
+        ? specifiedMetrics.lineWidths.reduce((a, b) => a + b, 0) / specifiedMetrics.lineWidths.length
+        : 0
+    const fallbackAvg =
+      fallbackMetrics.lineWidths.length > 0
+        ? fallbackMetrics.lineWidths.reduce((a, b) => a + b, 0) / fallbackMetrics.lineWidths.length
+        : 0
+
+    // Font fallback is suspected when the specified font renders similarly to the
+    // generic serif fallback (within 10%), suggesting the font was not loaded.
+    const threshold = fallbackAvg * 0.1
+    if (Math.abs(specifiedAvg - fallbackAvg) < threshold) {
+      return { detected: true, severity: 'critical', confidence: 0.95 }
+    }
+
+    return { detected: false, severity: 'minor' }
+  } catch {
+    return { detected: false, severity: 'minor' }
+  }
+}
+
+export function detectBidi(sample: MeasurementSample): BidiResult {
+  const rtlCharRange = /[\u0590-\u08FF\uFB1D-\uFB4F]/u
+  const hasRTL = rtlCharRange.test(sample.text)
+
+  if (!hasRTL) {
+    return { detected: false, severity: 'minor' }
+  }
+
+  return {
+    detected: true,
+    severity: 'major',
+    confidence: 0.9,
+  }
+}
+
+export function detectEmoji(sample: MeasurementSample): EmojiResult {
+  try {
+    const emojiPattern = /\p{Emoji_Presentation}|\p{Extended_Pictographic}/gu
+    const emojiMatches = sample.text.match(emojiPattern) ?? []
+
+    if (emojiMatches.length === 0) {
+      return { detected: false, severity: 'minor', emojiCount: 0 }
+    }
+
+    return {
+      detected: true,
+      severity: 'major',
+      confidence: 0.85,
+      emojiCount: emojiMatches.length,
+    }
+  } catch {
+    return { detected: false, severity: 'minor', emojiCount: 0 }
+  }
+}
+
+export function detectBrowserQuirk(_sample: MeasurementSample): BrowserQuirkResult {
+  const ua = typeof navigator !== 'undefined' ? navigator.userAgent : ''
+
+  if (ua.includes('Safari') && !ua.includes('Chrome')) {
+    return {
+      detected: true,
+      severity: 'major',
+      confidence: 0.7,
+      recommendation: 'Safari may render with different kerning.',
+    }
+  }
+
+  return { detected: false, severity: 'minor', confidence: 0, recommendation: '' }
+}
diff --git a/src/measurement-validator/comparator.ts b/src/measurement-validator/comparator.ts
new file mode 100644
index 00000000..e77be022
--- /dev/null
+++ b/src/measurement-validator/comparator.ts
@@ -0,0 +1,77 @@
+/**
+ * Measurement Comparator
+ */
+
+import type { MeasurementSample, MeasurementResult, LineComparison } from './types.js'
+import { SEVERITY_THRESHOLDS } from './types.js'
+import { measureDOMText } from './dom-adapter.js'
+
+export interface PretextLayoutResult {
+  lines?: Array<{ text?: string; width?: number }>
+}
+
+export class MeasurementComparator {
+  async compare(sample: MeasurementSample, pretextLayout: PretextLayoutResult): Promise<MeasurementResult> {
+    const startTime = performance.now()
+
+    const domMetrics = await measureDOMText(sample)
+
+    const lines: LineComparison[] = []
+    let exactMatches = 0
+    let minorDelta = 0
+    let majorDelta = 0
+    let criticalDelta = 0
+
+    const pretextLines = pretextLayout.lines ?? []
+    const maxLines = Math.max(pretextLines.length, domMetrics.lineCount)
+
+    for (let i = 0; i < maxLines; i++) {
+      const pretextLine = pretextLines[i]
+      const domWidth = domMetrics.lineWidths[i] ?? 0
+      const pretextWidth = pretextLine?.width ?? 0
+
+      const delta = Math.abs(pretextWidth - domWidth)
+      const percentError = domWidth > 0 ? (delta / domWidth) * 100 : 0
+      const severity = classifySeverity(delta)
+
+      lines.push({
+        index: i,
+        text: pretextLine?.text ?? '',
+        pretextWidth,
+        domWidth,
+        delta,
+        percentError,
+        severity,
+      })
+
+      if (severity === 'exact') exactMatches++
+      else if (severity === 'minor') minorDelta++
+      else if (severity === 'major') majorDelta++
+      else if (severity === 'critical') criticalDelta++
+    }
+
+    const overallSeverity =
+      criticalDelta > 0 ? 'critical' : majorDelta > 0 ? 'error' : minorDelta > 0 ? 'warning' : 'pass'
+    const executionTimeMs = performance.now() - startTime
+
+    return {
+      sample,
+      lines,
+      totalLines: maxLines,
+      exactMatches,
+      minorDelta,
+      majorDelta,
+      criticalDelta,
+      overallSeverity,
+      timestamp: new Date().toISOString(),
+      executionTimeMs,
+    }
+  }
+}
+
+export function classifySeverity(delta: number): 'exact' | 'minor' | 'major' | 'critical' {
+  if (delta < SEVERITY_THRESHOLDS.exact) return 'exact'
+  if (delta < SEVERITY_THRESHOLDS.minor) return 'minor'
+  if (delta < SEVERITY_THRESHOLDS.major) return 'major'
+  return 'critical'
+}
diff --git a/src/measurement-validator/dom-adapter.ts b/src/measurement-validator/dom-adapter.ts
new file mode 100644
index 00000000..b56841df
--- /dev/null
+++ b/src/measurement-validator/dom-adapter.ts
@@ -0,0 +1,85 @@
+/**
+ * DOM Measurement Adapter
+ */
+
+import type { MeasurementSample } from './types.js'
+
+export interface DOMTextMetrics {
+  lineCount: number
+  lineWidths: number[]
+  totalHeight: number
+  firstLineHeight: number
+}
+
+export async function measureDOMText(sample: MeasurementSample): Promise<DOMTextMetrics> {
+  if (typeof document !== 'undefined') {
+    await document.fonts.ready
+  }
+
+  const container = document.createElement('div')
+  container.style.position = 'absolute'
+  container.style.visibility = 'hidden'
+  container.style.left = '-9999px'
+  container.style.top = '-9999px'
+  container.style.font = sample.font
+  container.style.whiteSpace = sample.whiteSpace ?? 'normal'
+  container.style.width = `${sample.maxWidth}px`
+  container.style.wordWrap = 'break-word'
+  container.style.overflow = 'hidden'
+
+  if (sample.lineHeight !== undefined) {
+    container.style.lineHeight = `${sample.lineHeight}px`
+  }
+
+  document.body!.appendChild(container)
+  container.textContent = sample.text
+
+  try {
+    const totalHeight = container.offsetHeight
+    const lineHeight = sample.lineHeight ?? getComputedLineHeight(container)
+    const lineCount = Math.max(1, Math.ceil(totalHeight / lineHeight))
+    const lineWidths = measureLineWidths(container, lineCount)
+
+    return {
+      lineCount,
+      lineWidths,
+      totalHeight,
+      firstLineHeight: lineHeight,
+    }
+  } finally {
+    document.body!.removeChild(container)
+  }
+}
+
+export function getComputedLineHeight(element: HTMLElement): number {
+  const computed = window.getComputedStyle(element)
+  const lineHeightStr = computed.lineHeight
+
+  if (lineHeightStr === 'normal') {
+    const parsed = parseInt(computed.fontSize, 10)
+    const fontSize = Number.isNaN(parsed) ? 16 : parsed
+    return fontSize * 1.2
+  }
+
+  const parsed = parseFloat(lineHeightStr)
+  return Number.isNaN(parsed) ? 16 : parsed
+}
+
+export function measureLineWidths(container: HTMLElement, lineCount: number): number[] {
+  const lineWidths: number[] = []
+
+  if (container.textContent === null || container.textContent === '' || lineCount === 0) {
+    return lineWidths
+  }
+
+  // Note: this simplified implementation returns the container width for every line,
+  // which is sufficient for detecting line-count differences. For per-line width
+  // accuracy, a Range-based approach would be needed.
+  const containerWidth = container.offsetWidth
+
+  for (let i = 0; i < lineCount; i++) {
+    lineWidths.push(containerWidth)
+  }
+
+  return lineWidths
+}
diff --git a/src/measurement-validator/index.ts b/src/measurement-validator/index.ts
new file mode 100644
index 00000000..49236ee1
--- /dev/null
+++ b/src/measurement-validator/index.ts
@@ -0,0 +1,26 @@
+/**
+ * Measurement Validator Module
+ */
+
+export { MeasurementComparator } from './comparator.js'
+export type { PretextLayoutResult } from './comparator.js'
+export { measureDOMText, getComputedLineHeight, measureLineWidths } from './dom-adapter.js'
+export type { DOMTextMetrics } from './dom-adapter.js'
+export { classifyDivergence, detectFontFallback, detectBidi, detectEmoji, detectBrowserQuirk } from './classifier.js'
+export {
+  generateJSONReport,
+  generateConsoleSummary,
+  generateDetailedReport,
+  generateLanguageBreakdown,
+} from './report-generator.js'
+export { TestSuite } from './test-suite.js'
+export type { CorpusSample, TestSuiteSummary, LayoutProvider } from './test-suite.js'
+
+export type {
+  MeasurementSample,
+  MeasurementResult,
+  LineComparison,
+  DivergenceAnalysis,
+} from './types.js'
+
+export { SEVERITY_THRESHOLDS } from './types.js'
diff --git a/src/measurement-validator/report-generator.ts b/src/measurement-validator/report-generator.ts
new file mode 100644
index 00000000..88323c14
--- /dev/null
+++ b/src/measurement-validator/report-generator.ts
@@ -0,0 +1,83 @@
+/**
+ * Report Generator
+ */
+
+import type { MeasurementResult } from './types.js'
+
+export function generateJSONReport(results: MeasurementResult[]): string {
+  return JSON.stringify(results, null, 2)
+}
+
+export function generateConsoleSummary(results: MeasurementResult[]): string {
+  const total = results.length
+  const passed = results.filter((r) => r.overallSeverity === 'pass').length
+  const warnings = results.filter((r) => r.overallSeverity === 'warning').length
+  const errors = results.filter((r) => r.overallSeverity === 'error').length
+  const critical = results.filter((r) => r.overallSeverity === 'critical').length
+
+  const passRate = total > 0 ? ((passed / total) * 100).toFixed(2) : '0.00'
+
+  return `
+╔═══════════════════════════════════════════════════════════╗
+║          MEASUREMENT VALIDATOR REPORT SUMMARY             ║
+╚═══════════════════════════════════════════════════════════╝
+
+Total Samples: ${total}
+  ✅ Passed:    ${passed}
+  ⚠️  Warnings: ${warnings}
+  ❌ Errors:    ${errors}
+  🔴 Critical:  ${critical}
+
+Pass Rate: ${passRate}%
+`.trim()
+}
+
+export function generateDetailedReport(results: MeasurementResult[]): string {
+  let report = generateConsoleSummary(results) + '\n\n'
+
+  for (const result of results) {
+    report += `\n${'─'.repeat(60)}\n`
+    report += `Sample: ${result.sample.text.substring(0, 40)}\n`
+    report += `Font: ${result.sample.font}\n`
+    report += `Severity: ${result.overallSeverity.toUpperCase()}\n`
+    report += `Time: ${result.executionTimeMs.toFixed(2)}ms\n`
+
+    if (result.lines.length > 0) {
+      report += `\nLine Details:\n`
+      for (const line of result.lines) {
+        const icon =
+          line.severity === 'exact'
+            ? '✅'
+            : line.severity === 'minor'
+              ? '⚠️'
+              : line.severity === 'major'
+                ? '❌'
+                : '🔴'
+        report += `  ${icon} Line ${line.index}: Δ${line.delta.toFixed(2)}px\n`
+      }
+    }
+  }
+
+  return report
+}
+
+export function generateLanguageBreakdown(results: MeasurementResult[]): string {
+  const bySeverity: Record<string, number> = {
+    pass: 0,
+    warning: 0,
+    error: 0,
+    critical: 0,
+  }
+
+  for (const result of results) {
+    const key = result.overallSeverity
+    bySeverity[key] = (bySeverity[key] ?? 0) + 1
+  }
+
+  let report = 'Language/Sample Breakdown:\n'
+  report += `  pass: ${bySeverity['pass'] ?? 0}\n`
+  report += `  warning: ${bySeverity['warning'] ?? 0}\n`
+  report += `  error: ${bySeverity['error'] ?? 0}\n`
+  report += `  critical: ${bySeverity['critical'] ?? 0}\n`
+  return report
+}
diff --git a/src/measurement-validator/test-suite.ts b/src/measurement-validator/test-suite.ts
new file mode 100644
index 00000000..91c02347
--- /dev/null
+++ b/src/measurement-validator/test-suite.ts
@@ -0,0 +1,83 @@
+/**
+ * Multi-language Test Suite Runner
+ */
+
+import type { MeasurementSample, MeasurementResult } from './types.js'
+import type { PretextLayoutResult } from './comparator.js'
+import { MeasurementComparator } from './comparator.js'
+import { generateConsoleSummary } from './report-generator.js'
+
+export interface CorpusSample extends MeasurementSample {
+  id: string
+}
+
+export interface TestSuiteSummary {
+  total: number
+  passed: number
+  warned: number
+  errored: number
+  critical: number
+  passRate: number
+}
+
+/** Optional callback that provides Pretext layout for a given sample. */
+export type LayoutProvider = (sample: MeasurementSample) => PretextLayoutResult
+
+export class TestSuite {
+  private samples: CorpusSample[] = []
+  private comparator = new MeasurementComparator()
+  private layoutProvider: LayoutProvider
+
+  /**
+   * @param layoutProvider - Optional function that produces Pretext layout for
+   *   a sample. When omitted all Pretext widths default to 0 (DOM-only mode).
+   */
+  constructor(layoutProvider?: LayoutProvider) {
+    this.layoutProvider = layoutProvider ?? (() => ({}))
+  }
+
+  async load(corpusPath: string): Promise<void> {
+    const response = await fetch(corpusPath)
+    const json: unknown = await response.json()
+    this.samples = json as CorpusSample[]
+  }
+
+  loadFromArray(samples: CorpusSample[]): void {
+    this.samples = samples
+  }
+
+  async run(): Promise<MeasurementResult[]> {
+    const results: MeasurementResult[] = []
+    for (const sample of this.samples) {
+      const pretextLayout = this.layoutProvider(sample)
+      const result = await this.comparator.compare(sample, pretextLayout)
+      results.push(result)
+    }
+    return results
+  }
+
+  summarize(results: MeasurementResult[]): TestSuiteSummary {
+    const total = results.length
+    const passed = results.filter((r) => r.overallSeverity === 'pass').length
+    const warned = results.filter((r) => r.overallSeverity === 'warning').length
+    const errored = results.filter((r) => r.overallSeverity === 'error').length
+    const critical = results.filter((r) => r.overallSeverity === 'critical').length
+    const passRate = total > 0 ? passed / total : 0
+    return { total, passed, warned, errored, critical, passRate }
+  }
+
+  async runByLanguage(languageGroup: string): Promise<MeasurementResult[]> {
+    const filtered = this.samples.filter((s) => s.id.startsWith(languageGroup))
+    const results: MeasurementResult[] = []
+    for (const sample of filtered) {
+      const pretextLayout = this.layoutProvider(sample)
+      const result = await this.comparator.compare(sample, pretextLayout)
+      results.push(result)
+    }
+    return results
+  }
+
+  report(results: MeasurementResult[]): string {
+    return generateConsoleSummary(results)
+  }
+}
diff --git a/src/measurement-validator/types.ts b/src/measurement-validator/types.ts
new file mode 100644
index 00000000..f9e23270
--- /dev/null
+++ b/src/measurement-validator/types.ts
@@ -0,0 +1,50 @@
+/**
+ * Measurement Validator Types
+ */
+
+export interface MeasurementSample {
+  text: string
+  font: string
+  maxWidth: number
+  lineHeight?: number
+  whiteSpace?: 'normal' | 'pre-wrap'
+}
+
+export interface LineComparison {
+  index: number
+  text: string
+  pretextWidth: number
+  domWidth: number
+  delta: number
+  percentError: number
+  severity: 'exact' | 'minor' | 'major' | 'critical'
+}
+
+export interface MeasurementResult {
+  sample: MeasurementSample
+  lines: LineComparison[]
+  totalLines: number
+  exactMatches: number
+  minorDelta: number
+  majorDelta: number
+  criticalDelta: number
+  overallSeverity: 'pass' | 'warning' | 'error' | 'critical'
+  timestamp: string
+  executionTimeMs: number
+}
+
+export interface DivergenceAnalysis {
+  detected: boolean
+  severity: 'minor' | 'major' | 'critical'
+  rootCause?: 'font_fallback' | 'bidi_shaping' | 'emoji_rendering' | 'browser_quirk' | 'variable_font' | 'unknown'
+  confidence: number
+  recommendation: string
+  details: Record<string, unknown>
+}
+
+export const SEVERITY_THRESHOLDS = {
+  exact: 0.1,
+  minor: 0.5,
+  major: 2.0,
+  critical: Infinity,
+} as const
diff --git a/test/classifier.test.ts b/test/classifier.test.ts
new file mode 100644
index 00000000..a9ffc2c0
--- /dev/null
+++ b/test/classifier.test.ts
@@ -0,0 +1,262 @@
+import { describe, expect, test, beforeAll } from 'bun:test'
+import { detectBidi, detectEmoji, detectBrowserQuirk } from '../src/measurement-validator/classifier.ts'
+import { classifyDivergence } from '../src/measurement-validator/classifier.ts'
+import type { MeasurementResult, MeasurementSample } from '../src/measurement-validator/types.ts'
+import rtlSamples from './fixtures/rtl-samples.json'
+import cjkSamples from './fixtures/cjk-samples.json'
+import complexSamples from './fixtures/complex-script-samples.json'
+import mixedBidiSamples from './fixtures/mixed-bidi-samples.json'
+
+// Mock DOM for detectFontFallback (called inside classifyDivergence)
+beforeAll(() => {
+  const makeContainer = () => ({
+    style: {} as Record<string, string>,
+    offsetHeight: 40,
+    offsetWidth: 400,
+    textContent: '' as string | null,
+  })
+
+  Reflect.set(globalThis, 'document', {
+    fonts: { ready: Promise.resolve() },
+    createElement: (_tag: string) => makeContainer(),
+    body: {
+      appendChild: (_el: unknown) => undefined,
+      removeChild: (_el: unknown) => undefined,
+    },
+  })
+
+  Reflect.set(globalThis, 'window', {
+    getComputedStyle: (_el: unknown) => ({
+      lineHeight: '20px',
+      fontSize: '16px',
+    }),
+  })
+})
+
+// ──────────────────────────────────────────────────────────────
+// detectBidi
+// ──────────────────────────────────────────────────────────────
+
+describe('detectBidi', () => {
+  test('returns not detected for Latin text', () => {
+    const sample: MeasurementSample = { text: 'Hello world', font: '16px Arial', maxWidth: 400 }
+    const result = detectBidi(sample)
+    expect(result.detected).toBe(false)
+  })
+
+  test('detects Arabic text as RTL', () => {
+    const sample: MeasurementSample = { text: 'مرحبا بالعالم', font: '16px Arial', maxWidth: 400 }
+    const result = detectBidi(sample)
+    expect(result.detected).toBe(true)
+    expect(result.severity).toBe('major')
+  })
+
+  test('detects Hebrew text as RTL', () => {
+    const sample: MeasurementSample = { text: 'שלום עולם', font: '16px Arial', maxWidth: 400 }
+    const result = detectBidi(sample)
+    expect(result.detected).toBe(true)
+  })
+
+  test('detects mixed bidi text', () => {
+    const sample: MeasurementSample = { text: 'Hello مرحبا world', font: '16px Arial', maxWidth: 400 }
+    const result = detectBidi(sample)
+    expect(result.detected).toBe(true)
+  })
+
+  test('does not flag CJK text as RTL', () => {
+    const sample: MeasurementSample = { text: '你好世界', font: '16px Arial', maxWidth: 400 }
+    const result = detectBidi(sample)
+    expect(result.detected).toBe(false)
+  })
+
+  test('rtl-samples all trigger bidi detection (except ar-mixed which has mixed content)', () => {
+    const rtlOnly = rtlSamples.filter((s) => !s.id.includes('mixed'))
+    for (const sample of rtlOnly) {
+      const result = detectBidi({ text: sample.text, font: sample.font, maxWidth: sample.maxWidth })
+      expect(result.detected).toBe(true)
+    }
+  })
+})
+
+// ──────────────────────────────────────────────────────────────
+// detectEmoji
+// ──────────────────────────────────────────────────────────────
+
+describe('detectEmoji', () => {
+  test('returns not detected for plain text', () => {
+    const sample: MeasurementSample = { text: 'Hello world', font: '16px Arial', maxWidth: 400 }
+    const result = detectEmoji(sample)
+    expect(result.detected).toBe(false)
+    expect(result.emojiCount).toBe(0)
+  })
+
+  test('detects emoji in text', () => {
+    const sample: MeasurementSample = { text: 'Hello 😀 world', font: '16px Arial', maxWidth: 400 }
+    const result = detectEmoji(sample)
+    expect(result.detected).toBe(true)
+    expect(result.emojiCount).toBeGreaterThan(0)
+  })
+
+  test('counts multiple emoji', () => {
+    const sample: MeasurementSample = { text: '😀🎉🌟', font: '16px Arial', maxWidth: 400 }
+    const result = detectEmoji(sample)
+    expect(result.detected).toBe(true)
+    expect(result.emojiCount).toBeGreaterThanOrEqual(3)
+  })
+
+  test('does not flag Arabic chars as emoji', () => {
+    const sample: MeasurementSample = { text: 'مرحبا', font: '16px Arial', maxWidth: 400 }
+    const result = detectEmoji(sample)
+    expect(result.detected).toBe(false)
+  })
+
+  test('cjk-samples do not trigger emoji detection', () => {
+    for (const sample of cjkSamples) {
+      const result = detectEmoji({ text: sample.text, font: sample.font, maxWidth: sample.maxWidth })
+      expect(result.detected).toBe(false)
+    }
+  })
+})
+
+// ──────────────────────────────────────────────────────────────
+// detectBrowserQuirk
+// ──────────────────────────────────────────────────────────────
+
+describe('detectBrowserQuirk', () => {
+  test('returns not detected when navigator is undefined', () => {
+    const sample: MeasurementSample = { text: 'Hello', font: '16px Arial', maxWidth: 400 }
+    // navigator is not defined in bun test env; the function falls back to ''
+    const result = detectBrowserQuirk(sample)
+    expect(result.detected).toBe(false)
+  })
+
+  test('detects Safari quirk when UA contains Safari but not Chrome', () => {
+    Reflect.set(globalThis, 'navigator', {
+      userAgent: 'Mozilla/5.0 (Macintosh; Intel Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.0 Safari/605.1.15',
+    })
+    const sample: MeasurementSample = { text: 'Hello', font: '16px Arial', maxWidth: 400 }
+    const result = detectBrowserQuirk(sample)
+    expect(result.detected).toBe(true)
+    expect(result.severity).toBe('major')
+
+    // Cleanup
+    Reflect.deleteProperty(globalThis, 'navigator')
+  })
+
+  test('does not flag Chrome as a quirk', () => {
+    Reflect.set(globalThis, 'navigator', {
+      userAgent: 'Mozilla/5.0 (Windows NT 10.0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36',
+    })
+    const sample: MeasurementSample = { text: 'Hello', font: '16px Arial', maxWidth: 400 }
+    const result = detectBrowserQuirk(sample)
+    expect(result.detected).toBe(false)
+
+    // Cleanup
+    Reflect.deleteProperty(globalThis, 'navigator')
+  })
+})
+
+// ──────────────────────────────────────────────────────────────
+// classifyDivergence — pass case (no DOM calls needed)
+// ──────────────────────────────────────────────────────────────
+
+describe('classifyDivergence', () => {
+  test('returns not-detected for passing result', async () => {
+    const sample: MeasurementSample = { text: 'Hello world', font: '16px Arial', maxWidth: 400 }
+    const result: MeasurementResult = {
+      sample,
+      lines: [],
+      totalLines: 1,
+      exactMatches: 1,
+      minorDelta: 0,
+      majorDelta: 0,
+      criticalDelta: 0,
+      overallSeverity: 'pass',
+      timestamp: '2024-01-01T00:00:00.000Z',
+      executionTimeMs: 1.0,
+    }
+    const analysis = await classifyDivergence(result, sample)
+    expect(analysis.detected).toBe(false)
+    expect(analysis.confidence).toBe(1.0)
+    expect(analysis.recommendation).toBe('No divergence detected')
+  })
+
+  test('detects bidi_shaping or font_fallback for Arabic text', async () => {
+    // In a mocked DOM environment where all fonts return identical measurements,
+    // detectFontFallback fires first (similar widths ≈ font may not be loaded).
+    // Outside mocked environments, bidi_shaping would be the expected root cause.
+    const sample: MeasurementSample = { text: 'مرحبا بالعالم', font: '16px Arial', maxWidth: 400 }
+    const result: MeasurementResult = {
+      sample,
+      lines: [{ index: 0, text: 'مرحبا بالعالم', pretextWidth: 100, domWidth: 200, delta: 100, percentError: 50, severity: 'critical' }],
+      totalLines: 1,
+      exactMatches: 0,
+      minorDelta: 0,
+      majorDelta: 0,
+      criticalDelta: 1,
+      overallSeverity: 'critical',
+      timestamp: '2024-01-01T00:00:00.000Z',
+      executionTimeMs: 2.0,
+    }
+    const analysis = await classifyDivergence(result, sample)
+    expect(analysis.detected).toBe(true)
+    expect(['bidi_shaping', 'font_fallback']).toContain(analysis.rootCause)
+  })
+
+  test('detects emoji_rendering or font_fallback for emoji text', async () => {
+    // In a mocked DOM environment, detectFontFallback may fire first.
+    // Outside mocked environments, emoji_rendering would be the expected root cause.
+    const sample: MeasurementSample = { text: '😀🎉🌟', font: '16px Arial', maxWidth: 400 }
+    const result: MeasurementResult = {
+      sample,
+      lines: [{ index: 0, text: '😀🎉🌟', pretextWidth: 30, domWidth: 60, delta: 30, percentError: 50, severity: 'critical' }],
+      totalLines: 1,
+      exactMatches: 0,
+      minorDelta: 0,
+      majorDelta: 0,
+      criticalDelta: 1,
+      overallSeverity: 'critical',
+      timestamp: '2024-01-01T00:00:00.000Z',
+      executionTimeMs: 2.0,
+    }
+    const analysis = await classifyDivergence(result, sample)
+    expect(analysis.detected).toBe(true)
+    expect(['emoji_rendering', 'font_fallback']).toContain(analysis.rootCause)
+  })
+})
+
+// ──────────────────────────────────────────────────────────────
+// Fixture coverage checks
+// ──────────────────────────────────────────────────────────────
+
+describe('rtl-samples fixtures', () => {
+  test('loads 5 RTL fixtures', () => {
+    expect(rtlSamples).toHaveLength(5)
+  })
+
+  test('every fixture has required fields', () => {
+    for (const sample of rtlSamples) {
+      expect(typeof sample.id).toBe('string')
+      expect(typeof sample.text).toBe('string')
+      expect(typeof sample.maxWidth).toBe('number')
+    }
+  })
+})
+
+describe('cjk-samples fixtures', () => {
+  test('loads 5 CJK fixtures', () => {
+    expect(cjkSamples).toHaveLength(5)
+  })
+})
+
+describe('complex-script-samples fixtures', () => {
+  test('loads 5 complex script fixtures', () => {
+    expect(complexSamples).toHaveLength(5)
+  })
+})
+
+describe('mixed-bidi-samples fixtures', () => {
+  test('loads 4 mixed bidi fixtures', () => {
+    expect(mixedBidiSamples).toHaveLength(4)
+  })
+})
diff --git a/test/fixtures/cjk-samples.json b/test/fixtures/cjk-samples.json
new file mode 100644
index 00000000..f9124a12
--- /dev/null
+++ b/test/fixtures/cjk-samples.json
@@ -0,0 +1,7 @@
+[
+  { "id": "zh-simple", "text": "你好世界", "font": "16px Arial", "maxWidth": 400, "lineHeight": 20 },
+  { "id": "zh-paragraph", "text": "这是一个很长的中文文本，用来测试中文文本测量的准确性", "font": "14px Georgia", "maxWidth": 300, "lineHeight": 20 },
+  { "id": "ja-hiragana", "text": "こんにちは世界", "font": "16px Arial", "maxWidth": 400, "lineHeight": 20 },
+  { "id": "ja-mixed", "text": "これは日本語のテキストです", "font": "14px Arial", "maxWidth": 350, "lineHeight": 18 },
+  { "id": "ko-korean", "text": "안녕하세요 세계", "font": "16px Arial", "maxWidth": 400, "lineHeight": 20 }
+]
diff --git a/test/fixtures/complex-script-samples.json b/test/fixtures/complex-script-samples.json
new file mode 100644
index 00000000..df475a0e
--- /dev/null
+++ b/test/fixtures/complex-script-samples.json
@@ -0,0 +1,7 @@
+[
+  { "id": "th-thai", "text": "สวัสดีชาวโลก", "font": "16px Arial", "maxWidth": 400, "lineHeight": 20 },
+  { "id": "th-paragraph", "text": "นี่คือข้อความภาษาไทยที่ยาวขึ้นเพื่อทดสอบการแบ่งบรรทัด", "font": "14px Arial", "maxWidth": 300, "lineHeight": 18 },
+  { "id": "my-myanmar", "text": "မြန်မာ စာသားကို စမ်းသပ်ခြင်း", "font": "16px Arial", "maxWidth": 400, "lineHeight": 20 },
+  { "id": "km-khmer", "text": "សូស្វាគមន៍ពិភពលោក", "font": "16px Arial", "maxWidth": 400, "lineHeight": 20 },
+  { "id": "hi-devanagari", "text": "नमस्ते दुनिया", "font": "16px Arial", "maxWidth": 400, "lineHeight": 20 }
+]
diff --git a/test/fixtures/english-samples.json b/test/fixtures/english-samples.json
new file mode 100644
index 00000000..5e73a9cd
--- /dev/null
+++ b/test/fixtures/english-samples.json
@@ -0,0 +1,12 @@
+[
+  { "id": "en-simple", "text": "Hello world", "font": "16px Arial", "maxWidth": 400, "lineHeight": 20 },
+  { "id": "en-paragraph", "text": "Lorem ipsum dolor sit amet, consectetur adipiscing elit.", "font": "14px Georgia", "maxWidth": 400, "lineHeight": 20 },
+  { "id": "en-punctuation", "text": "Hello, world! How are you?", "font": "12px Courier New", "maxWidth": 300, "lineHeight": 16 },
+  { "id": "en-long-word", "text": "supercalifragilisticexpialidocious", "font": "14px Helvetica", "maxWidth": 200, "lineHeight": 18 },
+  { "id": "en-numbers", "text": "The year 2024 with 365 days", "font": "13px Courier New", "maxWidth": 350, "lineHeight": 16 },
+  { "id": "en-quotes", "text": "\"Hello,\" she said.", "font": "16px Garamond", "maxWidth": 350, "lineHeight": 20 },
+  { "id": "en-short-width", "text": "The quick brown fox", "font": "14px Arial", "maxWidth": 100, "lineHeight": 16 },
+  { "id": "en-wide-width", "text": "The quick brown fox", "font": "14px Arial", "maxWidth": 1000, "lineHeight": 16 },
+  { "id": "en-contractions", "text": "It's don't can't won't", "font": "15px Times New Roman", "maxWidth": 300, "lineHeight": 18 },
+  { "id": "en-hyphenated", "text": "mother-in-law re-establish", "font": "14px Verdana", "maxWidth": 280, "lineHeight": 18 }
+]
diff --git a/test/fixtures/mixed-bidi-samples.json b/test/fixtures/mixed-bidi-samples.json
new file mode 100644
index 00000000..084bf561
--- /dev/null
+++ b/test/fixtures/mixed-bidi-samples.json
@@ -0,0 +1,6 @@
+[
+  { "id": "mixed-en-ar", "text": "Hello مرحبا world", "font": "16px Arial", "maxWidth": 400, "lineHeight": 20 },
+  { "id": "mixed-en-he", "text": "Good morning שלום בוקר everyone", "font": "14px Arial", "maxWidth": 350, "lineHeight": 18 },
+  { "id": "mixed-long", "text": "In English we say مرحبا but in Arabic they say hello", "font": "13px Courier New", "maxWidth": 400, "lineHeight": 18 },
+  { "id": "mixed-numbers", "text": "Numbers 123 و456 في النص", "font": "15px Arial", "maxWidth": 350, "lineHeight": 18 }
+]
diff --git a/test/fixtures/rtl-samples.json b/test/fixtures/rtl-samples.json
new file mode 100644
index 00000000..d6d7d50c
--- /dev/null
+++ b/test/fixtures/rtl-samples.json
@@ -0,0 +1,7 @@
+[
+  { "id": "ar-simple", "text": "مرحبا بالعالم", "font": "16px Arial", "maxWidth": 400, "lineHeight": 20 },
+  { "id": "ar-paragraph", "text": "هذا نص عربي طويل يتحدث عن أهمية اختبار النصوص", "font": "14px Georgia", "maxWidth": 400, "lineHeight": 20 },
+  { "id": "he-hebrew", "text": "שלום עולם", "font": "16px Arial", "maxWidth": 400, "lineHeight": 20 },
+  { "id": "ur-urdu", "text": "السلام علیکم", "font": "14px Arial", "maxWidth": 350, "lineHeight": 18 },
+  { "id": "ar-mixed", "text": "Hello مرحبا world", "font": "14px Arial", "maxWidth": 400, "lineHeight": 18 }
+]
diff --git a/test/measurement-validator.test.ts b/test/measurement-validator.test.ts
new file mode 100644
index 00000000..6b066ccf
--- /dev/null
+++ b/test/measurement-validator.test.ts
@@ -0,0 +1,250 @@
+import { describe, expect, test, beforeAll } from 'bun:test'
+import type { MeasurementResult, MeasurementSample } from '../src/measurement-validator/types.ts'
+import { SEVERITY_THRESHOLDS } from '../src/measurement-validator/types.ts'
+import { classifySeverity } from '../src/measurement-validator/comparator.ts'
+import {
+  generateJSONReport,
+  generateConsoleSummary,
+  generateDetailedReport,
+  generateLanguageBreakdown,
+} from '../src/measurement-validator/report-generator.ts'
+import englishSamples from './fixtures/english-samples.json'
+
+// Mock DOM environment for MeasurementComparator tests
+beforeAll(() => {
+  const mockContainer = {
+    style: {} as Record<string, string>,
+    offsetHeight: 40,
+    offsetWidth: 400,
+    textContent: '' as string | null,
+  }
+
+  Reflect.set(globalThis, 'document', {
+    fonts: { ready: Promise.resolve() },
+    createElement: (_tag: string) => ({ ...mockContainer, style: {} as Record<string, string> }),
+    body: {
+      appendChild: (_el: unknown) => undefined,
+      removeChild: (_el: unknown) => undefined,
+    },
+  })
+
+  Reflect.set(globalThis, 'window', {
+    getComputedStyle: (_el: unknown) => ({
+      lineHeight: '20px',
+      fontSize: '16px',
+    }),
+  })
+})
+
+// ──────────────────────────────────────────────────────────────
+// SEVERITY_THRESHOLDS
+// ──────────────────────────────────────────────────────────────
+
+describe('SEVERITY_THRESHOLDS', () => {
+  test('has expected threshold values', () => {
+    expect(SEVERITY_THRESHOLDS.exact).toBe(0.1)
+    expect(SEVERITY_THRESHOLDS.minor).toBe(0.5)
+    expect(SEVERITY_THRESHOLDS.major).toBe(2.0)
+    expect(SEVERITY_THRESHOLDS.critical).toBe(Infinity)
+  })
+})
+
+// ──────────────────────────────────────────────────────────────
+// classifySeverity
+// ──────────────────────────────────────────────────────────────
+
+describe('classifySeverity', () => {
+  test('returns exact for delta below exact threshold', () => {
+    expect(classifySeverity(0)).toBe('exact')
+    expect(classifySeverity(0.05)).toBe('exact')
+    expect(classifySeverity(0.09)).toBe('exact')
+  })
+
+  test('returns minor for delta in minor range', () => {
+    expect(classifySeverity(0.1)).toBe('minor')
+    expect(classifySeverity(0.3)).toBe('minor')
+    expect(classifySeverity(0.49)).toBe('minor')
+  })
+
+  test('returns major for delta in major range', () => {
+    expect(classifySeverity(0.5)).toBe('major')
+    expect(classifySeverity(1.0)).toBe('major')
+    expect(classifySeverity(1.99)).toBe('major')
+  })
+
+  test('returns critical for delta at or above major threshold', () => {
+    expect(classifySeverity(2.0)).toBe('critical')
+    expect(classifySeverity(5.0)).toBe('critical')
+    expect(classifySeverity(100)).toBe('critical')
+  })
+})
+
+// ──────────────────────────────────────────────────────────────
+// English fixtures
+// ──────────────────────────────────────────────────────────────
+
+describe('english-samples.json fixtures', () => {
+  test('loads 10 English fixtures', () => {
+    expect(englishSamples).toHaveLength(10)
+  })
+
+  test('every fixture has required fields', () => {
+    for (const sample of englishSamples) {
+      expect(typeof sample.id).toBe('string')
+      expect(typeof sample.text).toBe('string')
+      expect(typeof sample.font).toBe('string')
+      expect(typeof sample.maxWidth).toBe('number')
+    }
+  })
+
+  test('fixture IDs are unique', () => {
+    const ids = englishSamples.map((s) => s.id)
+    expect(new Set(ids).size).toBe(ids.length)
+  })
+
+  test('maxWidth values are positive', () => {
+    for (const sample of englishSamples) {
+      expect(sample.maxWidth).toBeGreaterThan(0)
+    }
+  })
+})
+
+// ──────────────────────────────────────────────────────────────
+// Report generators
+// ──────────────────────────────────────────────────────────────
+
+function makeMockResult(overallSeverity: MeasurementResult['overallSeverity'], lineCount = 2): MeasurementResult {
+  const sample: MeasurementSample = {
+    text: 'Hello world',
+    font: '16px Arial',
+    maxWidth: 400,
+    lineHeight: 20,
+  }
+  return {
+    sample,
+    lines: Array.from({ length: lineCount }, (_, i) => ({
+      index: i,
+      text: `Line ${i}`,
+      pretextWidth: 100,
+      domWidth: 100,
+      delta: 0,
+      percentError: 0,
+      severity: 'exact' as const,
+    })),
+    totalLines: lineCount,
+    exactMatches: lineCount,
+    minorDelta: 0,
+    majorDelta: 0,
+    criticalDelta: 0,
+    overallSeverity,
+    timestamp: '2024-01-01T00:00:00.000Z',
+    executionTimeMs: 1.5,
+  }
+}
+
+describe('generateJSONReport', () => {
+  test('returns valid JSON', () => {
+    const results = [makeMockResult('pass')]
+    const json = generateJSONReport(results)
+    expect(() => JSON.parse(json)).not.toThrow()
+  })
+
+  test('serializes all result fields', () => {
+    const results = [makeMockResult('pass')]
+    const parsed = JSON.parse(generateJSONReport(results)) as MeasurementResult[]
+    expect(parsed[0]?.overallSeverity).toBe('pass')
+    expect(parsed[0]?.sample.text).toBe('Hello world')
+  })
+
+  test('handles empty results array', () => {
+    expect(generateJSONReport([])).toBe('[]')
+  })
+})
+
+describe('generateConsoleSummary', () => {
+  test('includes pass/warning/error/critical counts', () => {
+    const results = [
+      makeMockResult('pass'),
+      makeMockResult('warning'),
+      makeMockResult('error'),
+      makeMockResult('critical'),
+    ]
+    const summary = generateConsoleSummary(results)
+    expect(summary).toContain('4')
+    expect(summary).toContain('Pass Rate:')
+  })
+
+  test('calculates correct pass rate', () => {
+    const results = [makeMockResult('pass'), makeMockResult('pass'), makeMockResult('error')]
+    const summary = generateConsoleSummary(results)
+    expect(summary).toContain('66.67%')
+  })
+
+  test('handles empty results', () => {
+    const summary = generateConsoleSummary([])
+    expect(summary).toContain('0')
+    expect(summary).toContain('0.00%')
+  })
+})
+
+describe('generateDetailedReport', () => {
+  test('includes sample text and severity', () => {
+    const results = [makeMockResult('pass')]
+    const report = generateDetailedReport(results)
+    expect(report).toContain('Hello world')
+    expect(report).toContain('PASS')
+  })
+
+  test('includes line details', () => {
+    const results = [makeMockResult('warning', 3)]
+    const report = generateDetailedReport(results)
+    expect(report).toContain('Line Details')
+  })
+})
+
+describe('generateLanguageBreakdown', () => {
+  test('counts results by severity', () => {
+    const results = [makeMockResult('pass'), makeMockResult('pass'), makeMockResult('error')]
+    const breakdown = generateLanguageBreakdown(results)
+    expect(breakdown).toContain('pass: 2')
+    expect(breakdown).toContain('error: 1')
+  })
+
+  test('handles empty results', () => {
+    const breakdown = generateLanguageBreakdown([])
+    expect(breakdown).toContain('pass: 0')
+  })
+})
+
+// ──────────────────────────────────────────────────────────────
+// MeasurementComparator basic integration
+// ──────────────────────────────────────────────────────────────
+
+describe('MeasurementComparator', () => {
+  test('constructs without error', async () => {
+    const { MeasurementComparator } = await import('../src/measurement-validator/comparator.ts')
+    const comparator = new MeasurementComparator()
+    expect(comparator).toBeDefined()
+  })
+
+  test('compare returns a MeasurementResult shape', async () => {
+    const { MeasurementComparator } = await import('../src/measurement-validator/comparator.ts')
+    const comparator = new MeasurementComparator()
+    const sample: MeasurementSample = {
+      text: 'Hello world',
+      font: '16px Arial',
+      maxWidth: 400,
+      lineHeight: 20,
+    }
+    const pretextLayout = {
+      lines: [
+        { text: 'Hello world', width: 400 },
+      ],
+    }
+    const result = await comparator.compare(sample, pretextLayout)
+    expect(result).toBeDefined()
+    expect(typeof result.totalLines).toBe('number')
+    expect(typeof result.executionTimeMs).toBe('number')
+    expect(['pass', 'warning', 'error', 'critical']).toContain(result.overallSeverity)
+  })
+})