diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e1f70e6..7abf770 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -36,3 +36,6 @@ jobs:
 
       - name: Build
         run: pnpm run build
+
+      - name: Verify Tailwind CSS compiled
+        run: pnpm run verify:css
diff --git a/package.json b/package.json
index 587c527..d29a21c 100644
--- a/package.json
+++ b/package.json
@@ -9,6 +9,7 @@
     "start": "next start",
     "lint": "pnpm exec eslint .",
     "typecheck": "tsc --noEmit",
+    "verify:css": "node scripts/assert-tailwind-css.mjs",
     "format": "prettier --write ."
   },
   "keywords": [
diff --git a/postcss.config.mjs b/postcss.config.mjs
new file mode 100644
index 0000000..a869506
--- /dev/null
+++ b/postcss.config.mjs
@@ -0,0 +1,8 @@
+/** @type {import('postcss-load-config').Config} */
+const config = {
+  plugins: {
+    '@tailwindcss/postcss': {},
+  },
+}
+
+export default config
diff --git a/scripts/assert-tailwind-css.mjs b/scripts/assert-tailwind-css.mjs
new file mode 100644
index 0000000..383c9cd
--- /dev/null
+++ b/scripts/assert-tailwind-css.mjs
@@ -0,0 +1,64 @@
+// Asserts that the production build emitted compiled Tailwind v4 utility CSS.
+//
+// This is a regression guard for the bug fixed in PR #92 / issue #84: without a
+// PostCSS config registering `@tailwindcss/postcss`, the `@import "tailwindcss"`
+// directive in app/globals.css is never processed, so the build emits HTML with
+// no Tailwind utility classes and the UI renders completely unstyled.
+//
+// Run AFTER `next build`. Scans the .next output for known utility classes that
+// must be present whenever Tailwind v4 compilation is working.
+
+import { readdirSync, readFileSync, statSync } from 'node:fs'
+import { join } from 'node:path'
+
+const BUILD_DIR = '.next'
+
+// Utilities used by the app's own components — if PostCSS processed Tailwind,
+// these compile into real rules. If the config is missing, they are absent.
+const REQUIRED_UTILITIES = ['.flex', '.grid', '.items-center', '.p-4', '.text-sm']
+
+function collectCssFiles(dir) {
+  const out = []
+  for (const entry of readdirSync(dir)) {
+    const full = join(dir, entry)
+    if (statSync(full).isDirectory()) {
+      out.push(...collectCssFiles(full))
+    } else if (entry.endsWith('.css')) {
+      out.push(full)
+    }
+  }
+  return out
+}
+
+function fail(message) {
+  console.error(`\n[assert-tailwind-css] FAIL: ${message}\n`)
+  process.exit(1)
+}
+
+let cssFiles
+try {
+  cssFiles = collectCssFiles(BUILD_DIR)
+} catch {
+  fail(`build output "${BUILD_DIR}" not found — run \`pnpm build\` first.`)
+}
+
+if (cssFiles.length === 0) {
+  fail('no CSS files emitted by the build — Tailwind/PostCSS is not running.')
+}
+
+const allCss = cssFiles.map((f) => readFileSync(f, 'utf8')).join('\n')
+
+const missing = REQUIRED_UTILITIES.filter((cls) => !allCss.includes(`${cls}{`) && !allCss.includes(`${cls} `) && !allCss.includes(`${cls},`) && !allCss.includes(`${cls}:`))
+
+if (missing.length > 0) {
+  fail(
+    `compiled CSS is missing expected Tailwind utilities: ${missing.join(', ')}.\n` +
+      'This usually means postcss.config.mjs is missing or not registering ' +
+      '@tailwindcss/postcss, so `@import "tailwindcss"` was never processed.',
+  )
+}
+
+console.log(
+  `[assert-tailwind-css] OK: found Tailwind utilities (${REQUIRED_UTILITIES.join(', ')}) ` +
+    `in ${cssFiles.length} emitted CSS file(s).`,
+)