`hono-email` makes it straightforward to test email output. Strict-mode errors already reject `render()`, so unsupported tags or unsafe CSS fail tests automatically. Warnings require a small amount of configuration if you want them to fail tests.

## Fail on warnings

Pass `onWarning: 'error'` so `render()` throws when any warning is collected:

```ts
import { expect, test } from 'vitest'
import { render } from 'hono-email'
import WelcomeEmail from './welcome-email'

// Fail the test on any compatibility warning.
const renderEmail = (jsx: Parameters<typeof render>[0]) => render(jsx, { onWarning: 'error' })

test('welcome email renders without warnings', async () => {
  await expect(renderEmail(<WelcomeEmail />)).resolves.toBeDefined()
})
```

## Assert on warnings directly

Use `onWarning: 'silent'` to inspect `result.warnings` without console output:

```ts
test('welcome email has no warnings', async () => {
  const { warnings } = await render(<WelcomeEmail />, { onWarning: 'silent' })
  expect(warnings).toEqual([])
})
```

## Custom warning collector

You can also pass a callback to route warnings into your own collector:

```ts
const collected: Warning[] = []
await render(<WelcomeEmail />, {
  onWarning: (warning) => collected.push(warning),
})
```