diff --git a/.github/workflows/js.yml b/.github/workflows/js.yml
index 180f7c2..107ab7b 100644
--- a/.github/workflows/js.yml
+++ b/.github/workflows/js.yml
@@ -86,8 +86,50 @@ jobs:
     with:
       platform: js
 
-  release:
+  test:
     needs: build
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - { os: ubuntu-24.04 }
+          - { os: ubuntu-24.04-arm }
+          - { os: macos-26 }
+          - { os: windows-2025 }
+    steps:
+      - uses: actions/checkout@v5
+
+      - uses: actions/setup-node@v6
+        with:
+          node-version: 24.x
+
+      - name: Download artifact
+        uses: actions/download-artifact@v5
+        with:
+          path: build/js
+
+      - name: Install dependencies
+        run: |
+          curl -LO --output-dir cache https://github.com/fcitx-contrib/fcitx5-online/releases/download/latest/fcitx5-online.zip
+          unzip cache/fcitx5-online.zip
+          npm i -g pnpm
+          pnpm i
+          npx playwright install
+          npx playwright install-deps
+
+      - name: Test
+        if: ${{ startsWith(matrix.os, 'macos') || startsWith(matrix.os, 'windows') }}
+        run: pnpm run test
+
+      - name: Test
+        if: ${{ startsWith(matrix.os, 'ubuntu') }}
+        run: |
+          pnpm run test:chromium
+          pnpm run test:firefox
+
+  release:
+    needs: test
     if: ${{ github.ref == 'refs/heads/master' }}
     runs-on: ubuntu-latest
     steps:
diff --git a/.gitignore b/.gitignore
index dd5b3dc..64b6931 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,3 +5,8 @@ cache/js
 meta-*.json
 *.gz
 *.bz2
+node_modules
+pnpm-lock.yaml
+*.tsbuildinfo
+fcitx5-online*
+test-results
diff --git a/eslint.config.mjs b/eslint.config.mjs
new file mode 100644
index 0000000..0aa96e7
--- /dev/null
+++ b/eslint.config.mjs
@@ -0,0 +1,3 @@
+import antfu from '@antfu/eslint-config'
+
+export default antfu({})
diff --git a/package.json b/package.json
new file mode 100644
index 0000000..035e8c2
--- /dev/null
+++ b/package.json
@@ -0,0 +1,20 @@
+{
+  "name": "fcitx5-plugins",
+  "type": "module",
+  "scripts": {
+    "lint": "eslint tests",
+    "lint:fix": "eslint --fix tests",
+    "check": "tsc --noEmit",
+    "test": "playwright test --browser all",
+    "test:chromium": "playwright test --browser chromium",
+    "test:firefox": "playwright test --browser firefox",
+    "test:webkit": "playwright test --browser webkit"
+  },
+  "devDependencies": {
+    "@antfu/eslint-config": "^6.0.0",
+    "@playwright/test": "^1.56.1",
+    "eslint": "^9.38.0",
+    "serve": "^14.2.5",
+    "typescript": "^5.9.3"
+  }
+}
diff --git a/playwright.config.ts b/playwright.config.ts
new file mode 100644
index 0000000..1d3a18c
--- /dev/null
+++ b/playwright.config.ts
@@ -0,0 +1,11 @@
+import { defineConfig } from '@playwright/test'
+
+export default defineConfig({
+  testDir: 'tests',
+  fullyParallel: true,
+  webServer: {
+    command: 'npx serve -l 9000 -S fcitx5-online',
+    port: 9000,
+    reuseExistingServer: true,
+  },
+})
diff --git a/tests/test-mozc.spec.ts b/tests/test-mozc.spec.ts
new file mode 100644
index 0000000..a7bd3fd
--- /dev/null
+++ b/tests/test-mozc.spec.ts
@@ -0,0 +1,18 @@
+import { test } from '@playwright/test'
+import { expectCandidate, expectText, init } from './util'
+
+test('Mizu', async ({ page }) => {
+  test.skip(page.context().browser()!.browserType().name() === 'chromium', 'Chromium rejects big wasm executable.')
+  await init(page, 'mozc', 'Mozc')
+
+  await page.keyboard.type('m')
+  await page.keyboard.type('i')
+  await page.keyboard.type('z')
+  await page.keyboard.type('u')
+  await expectCandidate(page, '水')
+  await expectText(page, 'みず')
+
+  await page.keyboard.press(' ')
+  await page.keyboard.press('Enter')
+  await expectText(page, '水')
+})
diff --git a/tests/util.ts b/tests/util.ts
new file mode 100644
index 0000000..48dbac6
--- /dev/null
+++ b/tests/util.ts
@@ -0,0 +1,27 @@
+import type { Page } from '@playwright/test'
+import { expect } from '@playwright/test'
+
+export async function init(page: Page, plugin: string, im: string) {
+  await page.goto('http://localhost:9000')
+
+  // Install plugin.
+  await page.locator('.my-column > :first-child div:nth-child(6)').click()
+  const [fileChooser] = await Promise.all([
+    page.waitForEvent('filechooser'),
+    page.locator('.n-upload-trigger').click(),
+  ])
+  await fileChooser.setFiles([`build/js/${plugin}.zip`])
+  await page.keyboard.press('Escape')
+
+  // Select IM.
+  await page.locator('.my-column > :first-child .n-select').click()
+  await page.locator('.n-base-select-option').getByText(im).click()
+}
+
+export function expectCandidate(page: Page, text: string) {
+  return expect(page.locator('.fcitx-candidate').first().locator('.fcitx-text')).toHaveText(new RegExp(`^${text}$`))
+}
+
+export function expectText(page: Page, text: string) {
+  return expect(page.locator('.my-column > :nth-child(2) textarea')).toHaveValue(text)
+}
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..626af97
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,12 @@
+{
+  "compilerOptions": {
+    "composite": true,
+    "target": "ESNext",
+    "module": "ESNext",
+    "moduleResolution": "bundler",
+    "resolveJsonModule": true,
+    "allowSyntheticDefaultImports": true,
+    "skipLibCheck": true
+  },
+  "include": ["tests/**/*.ts"]
+}