Brotli: Browser Support, Algorithm, vs Gzip

What is Brotli?

Brotli is a general-purpose, lossless data compression algorithm that combines LZ77, Huffman coding, and second-order context modeling with a built-in static dictionary of around 13,000 web-friendly substrings. The IETF published it as RFC 7932, and browsers and servers use it for the Accept-Encoding: br HTTP content negotiation.

Which browsers does Brotli support?

Brotli is supported by every modern desktop and mobile browser through the Accept-Encoding: br HTTP header, with Chrome, Firefox, Opera, and Edge among the first to add it and Safari following on macOS and iOS.

How does Brotli compression work?

Brotli combines three techniques to squeeze more out of web text than older compression formats. The result is roughly 15 to 20% smaller files than Gzip on JavaScript, HTML, and CSS at the same fidelity. Three building blocks drive the savings:

• LZ77 backreferences: Brotli scans the input for repeated byte sequences and replaces later copies with short pointers back to the first match in a sliding window of up to 16 MB.
• Huffman coding: The encoder rewrites the resulting symbols so that frequent ones use fewer bits, the same trick Gzip uses but tuned for web content patterns.
• Second-order context modeling: Brotli predicts the next byte from the previous two bytes, letting the encoder pick a tighter Huffman table per context and shave more bits off the output.
• Static dictionary: Brotli ships with a 120 KiB dictionary of around 13,000 web-friendly substrings such as common HTML tags, CSS keywords, and JavaScript identifiers, so frequent patterns compress without filling the sliding window.

Brotli also exposes 12 compression levels, from 0 (fastest) to 11 (smallest output). Servers usually run levels 4 to 6 on dynamic responses and level 11 on cached static assets that are compressed once at build time.

How do you enable Brotli on your server?

Brotli is on by default in Cloudflare, Fastly, Akamai, and most major CDNs, but you may need to turn it on yourself for Nginx, Apache, or a custom origin. Follow these steps for the Nginx setup:

• Install the Nginx Brotli module: On Debian or Ubuntu, run sudo apt install libnginx-mod-http-brotli-filter libnginx-mod-http-brotli-static to add the runtime filter and the static-asset modules.
• Load the modules in nginx.conf: Add load_module modules/ngx_http_brotli_filter_module.so; and load_module modules/ngx_http_brotli_static_module.so; at the top of nginx.conf, above the events block.
• Configure compression in a server block: Inside the server block, add brotli on;, brotli_comp_level 6;, and a brotli_types line listing MIME types such as text/css, application/javascript, application/json, text/html, and image/svg+xml.
• Reload Nginx: Run sudo nginx -t to confirm the config parses, then sudo nginx -s reload so the new directives take effect without dropping live connections.
• Confirm Brotli is active: From your terminal, run curl -I -H "Accept-Encoding: br" https://your-site.com/ and look for content-encoding: br in the response headers.

If the response still comes back as content-encoding: gzip, the Brotli module did not load or the request URL is on plain HTTP. Re-check the load_module lines and confirm the request used HTTPS.

What are the known issues with Brotli?

Brotli ships in every modern browser, but a few real edge cases still bite in production. The biggest hits are HTTPS-only behavior, slow encoding at the top compression level, and uneven origin-server defaults.

• HTTPS-only in browsers: Chrome, Firefox, Edge, and Safari only advertise Accept-Encoding: br over HTTPS. Plain HTTP responses fall back to Gzip even when both client and server support Brotli.
• Slow at level 11: Brotli level 11 produces the smallest output but is roughly 50 to 100 times slower to encode than Gzip 6, so it is impractical for dynamic responses. Reserve level 11 for static assets compressed once at build time.
• Internet Explorer never supported it: IE 5.5 through 11 do not list br in Accept-Encoding. Sites that still serve IE traffic must keep a Gzip fallback in the response chain.
• Origin servers default to Gzip: Nginx, Apache, IIS, and Tomcat ship Gzip on by default but require manual setup for Brotli, so dev and staging environments often serve Gzip and mask the real-world payload sizes a CDN will produce.
• CDN re-compression varies by tier: Some CDNs strip Accept-Encoding: br on the way to the origin and re-compress at the edge, often at level 4. Static assets miss out on the level-11 savings unless you precompress and serve .br files directly.
• No standalone DecompressionStream support: The DecompressionStream API supports gzip and deflate-raw widely, but the br format is only standardized in Chromium-based browsers, so client-side Brotli decoding still needs a polyfill on Firefox and Safari.

In my experience, the most surprising failure is a CDN silently downgrading to level 4 on the edge while the origin pre-built level-11 files. The fix is to serve the precompressed .br files directly with brotli_static on; in Nginx so the CDN passes them through without re-encoding.

Citations

All Brotli version numbers and platform notes in this guide come from these primary sources:

• MDN Web Docs - Brotli compression: developer.mozilla.org/en-US/docs/Glossary/Brotli_compression
• MDN Web Docs - Accept-Encoding header: developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Encoding
• IETF RFC 7932 - Brotli Compressed Data Format: datatracker.ietf.org/doc/html/rfc7932
• Wikipedia - Brotli: en.wikipedia.org/wiki/Brotli
• Brotli reference implementation - GitHub: github.com/google/brotli
• The Brotli project site: brotli.org
• WebKit - New WebKit Features in Safari 11: webkit.org/blog/7551/new-webkit-features-in-safari-11
• Mozilla - Firefox 44 release notes: mozilla.org/en-US/firefox/44.0/releasenotes
• Chrome Platform Status - Brotli Content-Encoding: chromestatus.com/feature/5420797577396224
• Cloudflare blog - Brotli from origin: blog.cloudflare.com/this-is-brotli-from-origin
• Akamai docs - Brotli Support: techdocs.akamai.com/property-mgr/docs/brotli-support
• Nginx Brotli module - GitHub: github.com/google/ngx_brotli