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Rendertron Build status NPM rendertron package

Rendertron is a headless Chrome rendering solution designed to render & serialise web pages on the fly.

🔨 Built with Puppeteer

☁️ Easy deployment to Render

🔍 Improves SEO

Rendertron is designed to enable your Progressive Web App (PWA) to serve the correct content to any bot that doesn't render or execute JavaScript. Rendertron runs as a standalone HTTP server. Rendertron renders requested pages using Headless Chrome, auto-detecting when your PWA has completed loading and serializes the response back to the original request. To use Rendertron, your application configures middleware to determine whether to proxy a request to Rendertron. Rendertron is compatible with all client side technologies, including web components.

Demo endpoint

A demo Rendertron service is available at It is not designed to be used as a production endpoint. You can use it, but there are no uptime guarantees.



Once you have the service up and running, you'll need to implement the differential serving layer. This checks the user agent to determine whether prerendering is required.

This is a list of middleware available to use with the Rendertron service:

Rendertron is also compatible with middleware. Note: the user agent lists differ there.



GET /render/<url>

The render endpoint will render your page and serialize your page. Options are specified as query parameters:

  • mobile defaults to false. Enable by passing ?mobile to request the mobile version of your site.


GET /screenshot/<url>
POST /screenshot/<url>

The screenshot endpoint can be used to verify that your page is rendering correctly.

Both endpoints support the following query parameters:

  • width defaults to 1000 - specifies viewport width.
  • height defaults to 1000 - specifies viewport height.
  • mobile defaults to false. Enable by passing ?mobile to request the mobile version of your site.

Additional options are available as a JSON string in the POST body. See Puppeteer documentation for available options. You cannot specify the type (defaults to jpeg) and encoding (defaults to binary) parameters.


Query parameters

When setting query parameters as part of your URL, ensure they are encoded correctly. In JS, this would be encodeURIComponent(myURLWithParams). For example to specify page=home:

Auto detecting loading function

The service detects when a page has loaded by looking at the page load event, ensuring there are no outstanding network requests and that the page has had ample time to render.

Rendering budget timeout

There is a hard limit of 10 seconds for rendering. Ensure you don't hit this budget by ensuring your application is rendered well before the budget expires.

Web components

Headless Chrome supports web components but shadow DOM is difficult to serialize effectively. As such, shady DOM (a lightweight shim for Shadow DOM) is required for web components.

If you are using web components v0 (deprecated), you will need to enable Shady DOM to render correctly. In Polymer 1.x, which uses web components v0, Shady DOM is enabled by default. If you are using Shadow DOM, override this by setting the query parameter dom=shady when directing requests to the Rendertron service.

If you are using web components v1 and either webcomponents-lite.js or webcomponents-loader.js, set the query parameter wc-inject-shadydom=true when directing requests to the Rendertron service. This renderer service will force the necessary polyfills to be loaded and enabled.

Status codes

Status codes from the initial requested URL are preserved. If this is a 200, or 304, you can set the HTTP status returned by the rendering service by adding a meta tag.

<meta name="render:status_code" content="404" />

Running locally

To install Rendertron and run it locally, first install Rendertron:

npm install -g rendertron

With Chrome installed on your machine run the Rendertron CLI:


Installing & deploying


Clone and install dependencies:

git clone
cd rendertron
npm install
npm run build

Running locally

With a local instance of Chrome installed, you can start the server locally:

npm run start

Deploying to Render


Deploying using Docker

Rendertron no longer includes a Docker file. Instead, refer to Puppeteer documentation on how to deploy run headless Chrome in Docker.


When deploying the service, set configuration variables by including a config.json in the root. Available configuration options:

  • datastoreCache default false - set to true to enable caching on Google Cloud using datastore
  • timeout default 10000 - set the timeout used to render the target page.
  • port default 3000 - set the port to use for running and listening the rendertron service. Note if process.env.PORT is set, it will be used instead.
  • width default 1000 - set the width (resolution) to be used for rendering the page.
  • height default 1000 - set the height (resolution) to be used for rendering the page.


If you're having troubles with getting Headless Chrome to run in your environment, refer to the troubleshooting guide for Puppeteer.


A Headless Chrome rendering solution







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