diff --git a/AGENTS.md b/AGENTS.md
index 5cb8243..89b2a6e 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -29,6 +29,9 @@ python3 -m http.server 8080
 # Oder mit Live-Reload
 npx live-server
 
+# Tests ausführen (im Browser)
+# Server starten, dann http://localhost:8080/tests/ öffnen
+
 # Git Push (Token in URL)
 git push origin master
 ```
@@ -54,7 +57,6 @@ js/
 │   ├── categories.js   # Kategorien Service (Baum, Übersetzungen)
 │   ├── locations.js    # Standorte Service (Geo-Suche)
 │   ├── conversations.js # Zero-Knowledge Chat (E2E verschlüsselt)
-│   ├── chat.js         # LocalStorage Chat (Legacy/Mock)
 │   ├── crypto.js       # NaCl Encryption
 │   ├── currency.js     # XMR/Fiat Umrechnung
 │   └── pow-captcha.js  # Proof-of-Work Captcha (Challenge/Verify)
@@ -81,6 +83,13 @@ css/
 assets/
 └── fonts/              # Self-hosted Fonts (Inter, Space Grotesk)
 
+tests/
+├── index.html          # Test-Runner UI (im Browser öffnen)
+├── test-runner.js      # Minimaler Test-Framework
+├── helpers.test.js     # Unit Tests für helpers.js
+├── i18n.test.js        # Unit Tests für i18n.js
+└── router.test.js      # Unit Tests für router.js
+
 locales/
 ├── de.json             # Deutsch (Fallback)
 ├── en.json
diff --git a/README.md b/README.md
index f503f65..c747c0a 100644
--- a/README.md
+++ b/README.md
@@ -93,6 +93,18 @@ npx live-server
 
 Öffne http://localhost:8080
 
+### Tests ausführen
+
+```bash
+# Server starten
+python3 -m http.server 8080
+
+# Im Browser öffnen
+# http://localhost:8080/tests/
+```
+
+Die Tests laufen im Browser und nutzen einen minimalen Test-Runner ohne externe Dependencies.
+
 ### Projektstruktur
 
 ```
@@ -129,6 +141,10 @@ dgray/
 │   ├── de.json             # Deutsch
 │   ├── en.json             # English
 │   └── fr.json             # Français
+├── tests/
+│   ├── index.html          # Test-Runner UI
+│   ├── test-runner.js      # Test-Framework
+│   └── *.test.js           # Unit Tests
 └── assets/
     └── fonts/              # Self-hosted Fonts
 ```
diff --git a/tests/helpers.test.js b/tests/helpers.test.js
new file mode 100644
index 0000000..0bb1b91
--- /dev/null
+++ b/tests/helpers.test.js
@@ -0,0 +1,157 @@
+/**
+ * Tests for js/utils/helpers.js
+ */
+
+import { test, describe, assertEquals, assertTrue } from './test-runner.js'
+import { escapeHTML, formatPrice, formatRelativeTime, debounce, truncate } from '../js/utils/helpers.js'
+
+describe('escapeHTML', () => {
+    test('escapes < and >', () => {
+        assertEquals(escapeHTML('<script>'), '&lt;script&gt;')
+    })
+
+    test('escapes quotes', () => {
+        assertEquals(escapeHTML('"hello"'), '&quot;hello&quot;')
+    })
+
+    test('escapes ampersand', () => {
+        assertEquals(escapeHTML('a & b'), 'a &amp; b')
+    })
+
+    test('escapes single quotes', () => {
+        assertEquals(escapeHTML("it's"), "it&#039;s")
+    })
+
+    test('handles null', () => {
+        assertEquals(escapeHTML(null), '')
+    })
+
+    test('handles undefined', () => {
+        assertEquals(escapeHTML(undefined), '')
+    })
+
+    test('handles numbers by converting to string', () => {
+        assertEquals(escapeHTML(123), '123')
+    })
+
+    test('handles complex XSS attempt', () => {
+        const xss = '<img src="x" onerror="alert(1)">'
+        assertTrue(!escapeHTML(xss).includes('<'))
+    })
+})
+
+describe('formatPrice', () => {
+    test('formats EUR correctly', () => {
+        assertEquals(formatPrice(99.5, 'EUR'), '€ 99.50')
+    })
+
+    test('formats USD correctly', () => {
+        assertEquals(formatPrice(99.5, 'USD'), '$ 99.50')
+    })
+
+    test('formats CHF correctly', () => {
+        assertEquals(formatPrice(99.5, 'CHF'), 'CHF 99.50')
+    })
+
+    test('formats XMR with 4 decimals', () => {
+        assertEquals(formatPrice(0.5, 'XMR'), '0.5000 ɱ')
+    })
+
+    test('handles null price', () => {
+        assertEquals(formatPrice(null, 'EUR'), '–')
+    })
+
+    test('handles undefined price', () => {
+        assertEquals(formatPrice(undefined, 'EUR'), '–')
+    })
+
+    test('defaults to EUR', () => {
+        assertEquals(formatPrice(10), '€ 10.00')
+    })
+
+    test('handles unknown currency', () => {
+        assertEquals(formatPrice(10, 'GBP'), 'GBP 10.00')
+    })
+})
+
+describe('formatRelativeTime', () => {
+    test('formats seconds ago', () => {
+        const date = new Date(Date.now() - 30000) // 30 seconds ago
+        const result = formatRelativeTime(date, 'en')
+        assertTrue(result.includes('second'))
+    })
+
+    test('formats minutes ago', () => {
+        const date = new Date(Date.now() - 5 * 60000) // 5 minutes ago
+        const result = formatRelativeTime(date, 'en')
+        assertTrue(result.includes('minute'))
+    })
+
+    test('formats hours ago', () => {
+        const date = new Date(Date.now() - 2 * 3600000) // 2 hours ago
+        const result = formatRelativeTime(date, 'en')
+        assertTrue(result.includes('hour'))
+    })
+
+    test('formats days ago', () => {
+        const date = new Date(Date.now() - 3 * 86400000) // 3 days ago
+        const result = formatRelativeTime(date, 'en')
+        assertTrue(result.includes('day'))
+    })
+
+    test('accepts string date', () => {
+        const date = new Date(Date.now() - 60000).toISOString()
+        const result = formatRelativeTime(date, 'en')
+        assertTrue(result.includes('minute'))
+    })
+
+    test('respects locale de', () => {
+        const date = new Date(Date.now() - 60000)
+        const result = formatRelativeTime(date, 'de')
+        assertTrue(result.includes('Minute'))
+    })
+})
+
+describe('truncate', () => {
+    test('truncates long strings', () => {
+        assertEquals(truncate('Hello World', 6), 'Hello…')
+    })
+
+    test('does not truncate short strings', () => {
+        assertEquals(truncate('Hi', 10), 'Hi')
+    })
+
+    test('handles exact length', () => {
+        assertEquals(truncate('Hello', 5), 'Hello')
+    })
+
+    test('handles null', () => {
+        assertEquals(truncate(null, 10), null)
+    })
+
+    test('handles empty string', () => {
+        assertEquals(truncate('', 10), '')
+    })
+
+    test('uses default maxLength of 100', () => {
+        const long = 'a'.repeat(150)
+        const result = truncate(long)
+        assertEquals(result.length, 100)
+        assertTrue(result.endsWith('…'))
+    })
+})
+
+describe('debounce', () => {
+    test('returns a function', () => {
+        const fn = debounce(() => {})
+        assertEquals(typeof fn, 'function')
+    })
+
+    test('delays execution', (done) => {
+        let called = false
+        const fn = debounce(() => { called = true }, 50)
+        
+        fn()
+        assertEquals(called, false)
+    })
+})
diff --git a/tests/i18n.test.js b/tests/i18n.test.js
new file mode 100644
index 0000000..4ee2dbe
--- /dev/null
+++ b/tests/i18n.test.js
@@ -0,0 +1,91 @@
+/**
+ * Tests for js/i18n.js
+ */
+
+import { test, describe, assertEquals, assertTrue } from './test-runner.js'
+import { i18n, t } from '../js/i18n.js'
+
+describe('i18n.getNestedValue', () => {
+    const obj = {
+        home: {
+            title: 'Welcome',
+            nested: {
+                deep: 'Deep Value'
+            }
+        },
+        simple: 'Simple Value'
+    }
+
+    test('gets top-level value', () => {
+        assertEquals(i18n.getNestedValue(obj, 'simple'), 'Simple Value')
+    })
+
+    test('gets nested value', () => {
+        assertEquals(i18n.getNestedValue(obj, 'home.title'), 'Welcome')
+    })
+
+    test('gets deeply nested value', () => {
+        assertEquals(i18n.getNestedValue(obj, 'home.nested.deep'), 'Deep Value')
+    })
+
+    test('returns undefined for missing key', () => {
+        assertEquals(i18n.getNestedValue(obj, 'missing'), undefined)
+    })
+
+    test('returns undefined for missing nested key', () => {
+        assertEquals(i18n.getNestedValue(obj, 'home.missing'), undefined)
+    })
+})
+
+describe('i18n.t (translation)', () => {
+    // Note: These tests require translations to be loaded
+    // They test the parameter interpolation logic
+
+    test('returns key if translation missing', () => {
+        const result = i18n.t('this.key.does.not.exist')
+        assertEquals(result, 'this.key.does.not.exist')
+    })
+})
+
+describe('i18n.getLocale', () => {
+    test('returns current locale', () => {
+        const locale = i18n.getLocale()
+        assertTrue(['de', 'en', 'fr'].includes(locale))
+    })
+})
+
+describe('i18n.getSupportedLocales', () => {
+    test('returns array of supported locales', () => {
+        const locales = i18n.getSupportedLocales()
+        assertTrue(Array.isArray(locales))
+        assertTrue(locales.includes('de'))
+        assertTrue(locales.includes('en'))
+        assertTrue(locales.includes('fr'))
+    })
+})
+
+describe('i18n.getLocaleDisplayName', () => {
+    test('returns Deutsch for de', () => {
+        assertEquals(i18n.getLocaleDisplayName('de'), 'Deutsch')
+    })
+
+    test('returns English for en', () => {
+        assertEquals(i18n.getLocaleDisplayName('en'), 'English')
+    })
+
+    test('returns Français for fr', () => {
+        assertEquals(i18n.getLocaleDisplayName('fr'), 'Français')
+    })
+
+    test('returns locale code for unknown locale', () => {
+        assertEquals(i18n.getLocaleDisplayName('xx'), 'xx')
+    })
+})
+
+describe('i18n.subscribe', () => {
+    test('returns unsubscribe function', () => {
+        const unsubscribe = i18n.subscribe(() => {})
+        assertEquals(typeof unsubscribe, 'function')
+        unsubscribe()
+    })
+})
diff --git a/tests/index.html b/tests/index.html
new file mode 100644
index 0000000..7795b33
--- /dev/null
+++ b/tests/index.html
@@ -0,0 +1,124 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>dgray.io - Tests</title>
+    <style>
+        :root {
+            --color-bg: #1a1a1a;
+            --color-text: #f0f0f0;
+            --color-passed: #4ade80;
+            --color-failed: #f87171;
+            --color-border: #333;
+        }
+
+        * {
+            box-sizing: border-box;
+            margin: 0;
+            padding: 0;
+        }
+
+        body {
+            font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
+            background: var(--color-bg);
+            color: var(--color-text);
+            padding: 2rem;
+            line-height: 1.6;
+        }
+
+        h1 {
+            margin-bottom: 1rem;
+            font-size: 1.5rem;
+        }
+
+        h2 {
+            font-size: 1.2rem;
+            margin-bottom: 0.5rem;
+        }
+
+        .test-summary {
+            background: #222;
+            padding: 1rem;
+            border-radius: 8px;
+            margin-bottom: 1rem;
+        }
+
+        .test-summary .passed {
+            color: var(--color-passed);
+        }
+
+        .test-summary .failed {
+            color: var(--color-failed);
+        }
+
+        .test-list {
+            list-style: none;
+        }
+
+        .test-list li {
+            padding: 0.5rem 1rem;
+            border-bottom: 1px solid var(--color-border);
+        }
+
+        .test-list li.passed {
+            color: var(--color-passed);
+        }
+
+        .test-list li.failed {
+            color: var(--color-failed);
+        }
+
+        .test-list li small {
+            display: block;
+            color: #888;
+            font-family: monospace;
+            margin-top: 0.25rem;
+        }
+
+        #results {
+            max-width: 800px;
+            margin: 0 auto;
+        }
+
+        .loading {
+            color: #888;
+        }
+    </style>
+</head>
+<body>
+    <div id="results">
+        <h1>🧪 dgray.io Test Suite</h1>
+        <p class="loading">Running tests...</p>
+    </div>
+
+    <script type="module">
+        import { renderResults } from './test-runner.js'
+
+        const container = document.getElementById('results')
+
+        // Run all test files
+        async function runTests() {
+            try {
+                await import('./helpers.test.js')
+                await import('./i18n.test.js')
+                await import('./router.test.js')
+
+                // Clear loading message and render results
+                container.querySelector('.loading').remove()
+                renderResults(container)
+            } catch (error) {
+                container.innerHTML = `
+                    <h1>🧪 dgray.io Test Suite</h1>
+                    <p style="color: var(--color-failed)">
+                        Error loading tests: ${error.message}
+                    </p>
+                    <pre style="color: #888; font-size: 0.8rem; margin-top: 1rem;">${error.stack}</pre>
+                `
+            }
+        }
+
+        runTests()
+    </script>
+</body>
+</html>
diff --git a/tests/router.test.js b/tests/router.test.js
new file mode 100644
index 0000000..e885f50
--- /dev/null
+++ b/tests/router.test.js
@@ -0,0 +1,115 @@
+/**
+ * Tests for js/router.js
+ */
+
+import { test, describe, assertEquals, assertDeepEquals, assertTrue } from './test-runner.js'
+import { router } from '../js/router.js'
+
+describe('router.parseHash', () => {
+    test('parses simple path', () => {
+        // Save original hash
+        const original = window.location.hash
+        
+        window.location.hash = '/home'
+        const result = router.parseHash()
+        assertEquals(result.path, '/home')
+        assertDeepEquals(result.params, {})
+        
+        // Restore
+        window.location.hash = original
+    })
+
+    test('parses path with query params', () => {
+        const original = window.location.hash
+        
+        window.location.hash = '/search?q=test&page=2'
+        const result = router.parseHash()
+        assertEquals(result.path, '/search')
+        assertEquals(result.params.q, 'test')
+        assertEquals(result.params.page, '2')
+        
+        window.location.hash = original
+    })
+
+    test('returns / for empty hash', () => {
+        const original = window.location.hash
+        
+        window.location.hash = ''
+        const result = router.parseHash()
+        assertEquals(result.path, '/')
+        
+        window.location.hash = original
+    })
+})
+
+describe('router.matchRoute', () => {
+    // Register test routes
+    router.register('/test', 'test-page')
+    router.register('/user/:id', 'user-page')
+    router.register('/post/:id/comment/:commentId', 'comment-page')
+
+    test('matches exact route', () => {
+        const match = router.matchRoute('/test')
+        assertEquals(match.componentTag, 'test-page')
+        assertDeepEquals(match.params, {})
+    })
+
+    test('matches route with single param', () => {
+        const match = router.matchRoute('/user/123')
+        assertEquals(match.componentTag, 'user-page')
+        assertEquals(match.params.id, '123')
+    })
+
+    test('matches route with multiple params', () => {
+        const match = router.matchRoute('/post/456/comment/789')
+        assertEquals(match.componentTag, 'comment-page')
+        assertEquals(match.params.id, '456')
+        assertEquals(match.params.commentId, '789')
+    })
+
+    test('returns null for unmatched route', () => {
+        const match = router.matchRoute('/unknown/path')
+        assertEquals(match, null)
+    })
+
+    test('does not match wrong segment count', () => {
+        const match = router.matchRoute('/user/123/extra')
+        assertEquals(match, null)
+    })
+})
+
+describe('router.register', () => {
+    test('returns router for chaining', () => {
+        const result = router.register('/chain-test', 'chain-page')
+        assertEquals(result, router)
+    })
+})
+
+describe('router.getCurrentRoute', () => {
+    test('returns null before navigation', () => {
+        // Note: May have a value if other tests triggered navigation
+        const route = router.getCurrentRoute()
+        assertTrue(route === null || typeof route === 'object')
+    })
+})
+
+describe('router.navigate', () => {
+    test('sets hash', () => {
+        const original = window.location.hash
+        
+        router.navigate('/nav-test')
+        assertTrue(window.location.hash.includes('nav-test'))
+        
+        window.location.hash = original
+    })
+
+    test('includes query params', () => {
+        const original = window.location.hash
+        
+        router.navigate('/nav-test', { foo: 'bar', num: '42' })
+        assertTrue(window.location.hash.includes('foo=bar'))
+        assertTrue(window.location.hash.includes('num=42'))
+        
+        window.location.hash = original
+    })
+})
diff --git a/tests/test-runner.js b/tests/test-runner.js
new file mode 100644
index 0000000..80a35b2
--- /dev/null
+++ b/tests/test-runner.js
@@ -0,0 +1,145 @@
+/**
+ * Minimal Test Runner for Browser
+ * Run tests by opening tests/index.html in browser
+ */
+
+const results = {
+    passed: 0,
+    failed: 0,
+    tests: []
+}
+
+/**
+ * Define a test
+ * @param {string} name - Test name
+ * @param {Function} fn - Test function
+ */
+export function test(name, fn) {
+    try {
+        fn()
+        results.passed++
+        results.tests.push({ name, status: 'passed' })
+        console.log(`✓ ${name}`)
+    } catch (error) {
+        results.failed++
+        results.tests.push({ name, status: 'failed', error: error.message })
+        console.error(`✗ ${name}`)
+        console.error(`  ${error.message}`)
+    }
+}
+
+/**
+ * Group tests
+ * @param {string} name - Group name
+ * @param {Function} fn - Function containing tests
+ */
+export function describe(name, fn) {
+    console.group(name)
+    fn()
+    console.groupEnd()
+}
+
+/**
+ * Assert equality
+ * @param {*} actual 
+ * @param {*} expected 
+ * @param {string} [message]
+ */
+export function assertEquals(actual, expected, message = '') {
+    if (actual !== expected) {
+        throw new Error(
+            message || `Expected ${JSON.stringify(expected)}, got ${JSON.stringify(actual)}`
+        )
+    }
+}
+
+/**
+ * Assert truthy
+ * @param {*} value 
+ * @param {string} [message]
+ */
+export function assertTrue(value, message = '') {
+    if (!value) {
+        throw new Error(message || `Expected truthy value, got ${value}`)
+    }
+}
+
+/**
+ * Assert falsy
+ * @param {*} value 
+ * @param {string} [message]
+ */
+export function assertFalse(value, message = '') {
+    if (value) {
+        throw new Error(message || `Expected falsy value, got ${value}`)
+    }
+}
+
+/**
+ * Assert throws
+ * @param {Function} fn 
+ * @param {string} [message]
+ */
+export function assertThrows(fn, message = '') {
+    try {
+        fn()
+        throw new Error(message || 'Expected function to throw')
+    } catch (e) {
+        if (e.message === (message || 'Expected function to throw')) {
+            throw e
+        }
+    }
+}
+
+/**
+ * Assert deep equality for objects/arrays
+ * @param {*} actual 
+ * @param {*} expected 
+ * @param {string} [message]
+ */
+export function assertDeepEquals(actual, expected, message = '') {
+    const actualStr = JSON.stringify(actual)
+    const expectedStr = JSON.stringify(expected)
+    if (actualStr !== expectedStr) {
+        throw new Error(
+            message || `Expected ${expectedStr}, got ${actualStr}`
+        )
+    }
+}
+
+/**
+ * Get test results
+ * @returns {Object}
+ */
+export function getResults() {
+    return { ...results }
+}
+
+/**
+ * Render results to DOM
+ * @param {HTMLElement} container 
+ */
+export function renderResults(container) {
+    const summary = document.createElement('div')
+    summary.className = 'test-summary'
+    summary.innerHTML = `
+        <h2>Test Results</h2>
+        <p class="passed">Passed: ${results.passed}</p>
+        <p class="failed">Failed: ${results.failed}</p>
+    `
+    
+    const list = document.createElement('ul')
+    list.className = 'test-list'
+    
+    results.tests.forEach(t => {
+        const li = document.createElement('li')
+        li.className = t.status
+        li.innerHTML = t.status === 'passed' 
+            ? `✓ ${t.name}`
+            : `✗ ${t.name}<br><small>${t.error}</small>`
+        list.appendChild(li)
+    })
+    
+    container.appendChild(summary)
+    container.appendChild(list)
+}