Vercel CDN Compression

Vercel helps reduce data transfer and improve performance by supporting both Gzip and Brotli compression. These algorithms are widely used to compress files, such as HTML, CSS, and JavaScript, to reduce their size and improve performance.

While gzip has been around for quite some time, brotli is a newer compression algorithm built by Google that best serves text compression. If your client supports brotli, it takes precedence over gzip because:

• brotli compressed JavaScript files are 14% smaller than gzip
• HTML files are 21% smaller than gzip
• CSS files are 17% smaller than gzip

brotli has an advantage over gzip since it uses a dictionary of common keywords on both the client and server-side, which gives a better compression ratio.

Many clients (e.g., browsers like Chrome, Firefox, and Safari) include the Accept-Encoding request header by default. This automatically enables compression for Vercel's CDN.

You can verify if a response was compressed by checking the Content-Encoding response header has a value of gzip or brotli.

The following clients may not include the Accept-Encoding header by default:

• Custom applications, such as Python scripts, Node.js servers, or other software that can send HTTP requests to your deployment
• HTTP libraries, such as http in Node.js, and networking tools, like curl or wget
• Older browsers. Check MDN's browser compatibility list to see if your client supports Accept-Encoding by default
• Bots and crawlers sometimes do not specify Accept-Encoding in their headers by default when visiting your deployment

When writing a client that doesn't run in a browser, for example a CLI, you will need to set the Accept-Encoding request header in your client code to opt into compression.

When the Accept-Encoding request header is present, only the following list of MIME types will be automatically compressed.

• json
• x-web-app-manifest+json
• geo+json
• manifest+json
• ld+json
• atom+xml
• rss+xml
• xhtml+xml
• xml
• rdf+xml
• javascript
• tar
• vnd.ms-fontobject
• wasm

• otf
• ttf

• svg+xml
• bmp
• x-icon

• cache-manifest
• css
• csv
• dns
• javascript
• plain
• markdown
• vcard
• calendar
• vnd.rim.location.xloc
• vtt
• x-component
• x-cross-domain-policy

The compression allowlist above is necessary to avoid accidentally increasing the size of non-compressible files, which can negatively impact performance.

For example, most image formats are already compressed such as JPEG, PNG, WebP, etc. If you want to compress an image even further, consider lowering the quality using Vercel Image Optimization.