`.
If your application has specific CSS targeting span such as `.container span`, upgrading to Next.js 12 might incorrectly match the wrapping element inside the `` component. You can avoid this by restricting the selector to a specific class such as `.container span.item` and updating the relevant component with that className, such as ``.
If your application has specific CSS targeting the `next/image` `
` tag, for example `.container div`, it may not match anymore. You can update the selector `.container span`, or preferably, add a new `
` wrapping the `` component and target that instead such as `.container .wrapper`.
The `className` prop is unchanged and will still be passed to the underlying `![]()
` element.
See the [documentation](/docs/pages/api-reference/components/image#styling-images) for more info.
#### HMR connection now uses a WebSocket
Previously, Next.js used a [server-sent events](https://developer.mozilla.org/docs/Web/API/Server-sent_events) connection to receive HMR events. Next.js 12 now uses a WebSocket connection.
In some cases when proxying requests to the Next.js dev server, you will need to ensure the upgrade request is handled correctly. For example, in `nginx` you would need to add the following configuration:
```nginx
location /_next/webpack-hmr {
proxy_pass http://localhost:3000/_next/webpack-hmr;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
}
```
If you are using Apache (2.x), you can add the following configuration to enable web sockets to the server. Review the port, host name and server names.
```
# ServerName yourwebsite.local
ServerName "${WEBSITE_SERVER_NAME}"
ProxyPass / http://localhost:3000/
ProxyPassReverse / http://localhost:3000/
# Next.js 12 uses websocket
RewriteEngine On
RewriteCond %{QUERY_STRING} transport=websocket [NC]
RewriteCond %{HTTP:Upgrade} websocket [NC]
RewriteCond %{HTTP:Connection} upgrade [NC]
RewriteRule /(.*) ws://localhost:3000/_next/webpack-hmr/$1 [P,L]
ProxyPass ws://localhost:3000/_next/webpack-hmr retry=0 timeout=30
ProxyPassReverse ws://localhost:3000/_next/webpack-hmr
```
For custom servers, such as `express`, you may need to use `app.all` to ensure the request is passed correctly, for example:
```js
app.all('/_next/webpack-hmr', (req, res) => {
nextjsRequestHandler(req, res)
})
```
#### Webpack 4 support has been removed
If you are already using webpack 5 you can skip this section.
Next.js has adopted webpack 5 as the default for compilation in Next.js 11. As communicated in the [webpack 5 upgrading documentation](/docs/messages/webpack5) Next.js 12 removes support for webpack 4.
If your application is still using webpack 4 using the opt-out flag, you will now see an error linking to the [webpack 5 upgrading documentation](/docs/messages/webpack5).
#### `target` option deprecated
If you do not have `target` in `next.config.js` you can skip this section.
The target option has been deprecated in favor of built-in support for tracing what dependencies are needed to run a page.
During `next build`, Next.js will automatically trace each page and its dependencies to determine all of the files that are needed for deploying a production version of your application.
If you are currently using the `target` option set to `serverless`, please read the [documentation on how to leverage the new output](/docs/pages/api-reference/config/next-config-js/output).