Strict mode is enabled by default in `render()`. It is meant to fail early on markup, CSS, and image formats that are unsupported or risky for HTML email clients.

`hono-email` uses dynamic compatibility data derived from [caniemail.com](https://www.caniemail.com/) to analyze your markup.

## Validation Scope

The validation engine checks the following items:

- **HTML Tags & Attributes**: Interactive or dangerous elements (like `<script>`, `<iframe>`) and attributes.
- **CSS Properties & At-Rules**: Unsupported declarations (like `display: grid`) or risky layouts.
- **Image Formats**: Compatibility checks for image formats such as `webp`, `avif`, `svg`, and `base64`.

## Global Compatibility Thresholds

By default, `hono-email` checks compatibility using global support ratios derived from [caniemail.com](https://www.caniemail.com/). Features that have extremely poor support across all clients will trigger strict-mode errors (blocking render) or compatibility warnings.

## Warning Clients Configuration

To get additional client-specific compatibility warnings for features that are unsupported or only partially supported in certain environments, configure the `warningClients` option:

```tsx
const result = await render(<MyEmail />, {
  strict: true,
  warningClients: ['gmail', 'apple-mail'], // Get extra warnings for Gmail and Apple Mail
})
```

By default, `warningClients` is `[]` (no additional client-specific warnings are generated unless configured).

## Errors & Warnings Behavior

- **Errors**: Hard failures (like interactive tags or missing required fields) reject the `render()` promise.
- **Warnings**: Features that have limited support or quirks on target clients produce compatibility warnings on `RenderResult.warnings`.

Warnings are surfaced according to the [`onWarning`](/core/render/#onwarning) option.

## Disabling strict mode

You can disable strict validation, but doing so means `hono-email` will no longer protect you from markup that is likely to break in email clients.

```tsx
const { html } = await render(<MyEmail />, { strict: false })
```