diff --git a/web/package.json b/web/package.json
index 97f72c4..ddbca0a 100644
--- a/web/package.json
+++ b/web/package.json
@@ -21,7 +21,8 @@
     "test:e2e": "playwright test",
     "test:e2e:ui": "playwright test --ui",
     "test:e2e:viewport": "playwright test e2e/viewport-matrix.spec.ts",
-    "test:e2e:overflow": "playwright test e2e/horizontal-overflow.spec.ts"
+    "test:e2e:overflow": "playwright test e2e/horizontal-overflow.spec.ts",
+    "audit:css": "node scripts/audit-css.mjs"
   },
   "dependencies": {
     "@bytelyst/api-client": "^0.1.6",
diff --git a/web/scripts/audit-css.mjs b/web/scripts/audit-css.mjs
new file mode 100644
index 0000000..78e4b4b
--- /dev/null
+++ b/web/scripts/audit-css.mjs
@@ -0,0 +1,129 @@
+#!/usr/bin/env node
+// Audit CSS files in src/ for:
+//   - Class selectors defined more than once (rule duplication / drift)
+//   - Per-class !important counts (specificity-fight indicators)
+//
+// Outputs a report to stdout. Exits 0 always; this is informational.
+//
+// Usage:
+//   node scripts/audit-css.mjs [path-or-glob...]
+//   node scripts/audit-css.mjs --threshold 2   # only show classes with >= N defs
+//   node scripts/audit-css.mjs --json          # machine-readable output
+//
+// See docs/ui/UI_AUDIT.md §5 #7 and Pattern G.
+
+import fs from 'node:fs';
+import path from 'node:path';
+import { argv, exit } from 'node:process';
+
+const args = argv.slice(2);
+const opts = { threshold: 2, json: false, paths: [] };
+for (let i = 0; i < args.length; i++) {
+  const a = args[i];
+  if (a === '--threshold' || a === '-t') opts.threshold = parseInt(args[++i], 10);
+  else if (a === '--json') opts.json = true;
+  else opts.paths.push(a);
+}
+if (opts.paths.length === 0) opts.paths = ['src/index.css', 'src/App.css', 'src/layout-fixes.css'];
+
+const SELECTOR_RE = /(^|[\s},])\s*\.([a-zA-Z_][\w-]*)\b/g;
+const RULE_BLOCK_RE = /([^{}]+?)\{([^{}]*)\}/gs;
+
+function readSafe(p) {
+  try { return fs.readFileSync(p, 'utf8'); } catch { return null; }
+}
+
+function lineOf(content, idx) {
+  return content.slice(0, idx).split('\n').length;
+}
+
+function audit(filePath) {
+  const content = readSafe(filePath);
+  if (content == null) return null;
+  // For each top-level rule block, collect class selectors and !important counts.
+  const defs = new Map(); // className -> [{file, line, importants}]
+  let m;
+  RULE_BLOCK_RE.lastIndex = 0;
+  while ((m = RULE_BLOCK_RE.exec(content)) !== null) {
+    const selector = m[1];
+    const body = m[2];
+    const ruleStartIdx = m.index;
+    const importants = (body.match(/!important/g) || []).length;
+    // Skip @media / @supports / @keyframes blocks for "duplicate" detection — they're
+    // legitimately repeated. We track them separately below.
+    if (/^\s*@/.test(selector)) continue;
+    // Extract class names from selector list.
+    const classes = new Set();
+    let s;
+    SELECTOR_RE.lastIndex = 0;
+    while ((s = SELECTOR_RE.exec(selector)) !== null) {
+      classes.add(s[2]);
+    }
+    for (const cls of classes) {
+      if (!defs.has(cls)) defs.set(cls, []);
+      defs.get(cls).push({ file: filePath, line: lineOf(content, ruleStartIdx), importants });
+    }
+  }
+  // Total !important
+  const totalImportant = (content.match(/!important/g) || []).length;
+  return { filePath, defs, totalImportant };
+}
+
+const reports = [];
+for (const p of opts.paths) {
+  const abs = path.resolve(p);
+  const r = audit(abs);
+  if (r) reports.push(r);
+}
+
+// Merge defs across files
+const merged = new Map();
+let totalImportant = 0;
+for (const r of reports) {
+  totalImportant += r.totalImportant;
+  for (const [cls, locs] of r.defs.entries()) {
+    if (!merged.has(cls)) merged.set(cls, []);
+    merged.get(cls).push(...locs);
+  }
+}
+
+// Filter to classes with >= threshold definitions
+const dups = [...merged.entries()]
+  .filter(([, locs]) => locs.length >= opts.threshold)
+  .sort((a, b) => b[1].length - a[1].length);
+
+const importantPerClass = [...merged.entries()]
+  .map(([cls, locs]) => [cls, locs.reduce((s, l) => s + l.importants, 0)])
+  .filter(([, n]) => n > 0)
+  .sort((a, b) => b[1] - a[1]);
+
+if (opts.json) {
+  console.log(JSON.stringify({
+    files: opts.paths,
+    totalImportant,
+    duplicateSelectors: dups.map(([cls, locs]) => ({ class: cls, count: locs.length, locations: locs })),
+    importantByClass: importantPerClass.map(([cls, n]) => ({ class: cls, count: n })),
+  }, null, 2));
+  exit(0);
+}
+
+console.log('=== CSS audit ===');
+console.log(`Files scanned: ${opts.paths.join(', ')}`);
+console.log(`Total !important declarations: ${totalImportant}`);
+console.log('');
+console.log(`Class selectors defined ${opts.threshold}+ times (top 30):`);
+console.log('  count  class');
+console.log('  -----  ----------------------------------------');
+for (const [cls, locs] of dups.slice(0, 30)) {
+  const lines = locs.map(l => `${path.basename(l.file)}:${l.line}`).join(', ');
+  console.log(`  ${String(locs.length).padStart(5)}  .${cls}  (${lines})`);
+}
+console.log('');
+console.log('Top 15 classes by !important count (each declaration is a specificity fight):');
+console.log('  count  class');
+console.log('  -----  ----------------------------------------');
+for (const [cls, n] of importantPerClass.slice(0, 15)) {
+  console.log(`  ${String(n).padStart(5)}  .${cls}`);
+}
+console.log('');
+console.log('See docs/ui/UI_AUDIT.md Pattern G for fix guidance.');