Brotli and Gzip: What modern websites really need

When should a modern website offer Brotli, Gzip, or both compression methods?

Modern websites offer Brotli for supported clients and Gzip as a broad fallback for suitable text content. The selection is made via Accept-Encoding; the cache and CDN must correctly distinguish between variants using Vary.

Test case: "Missing Vary header"

A CDN delivers JavaScript via Brotli but retains a Gzip variant for an older client. Images remain in their optimized format; a test with varying Accept-Encoding confirms that the cache does not incorrectly reuse any Brotli responses.

Clean negotiation

1. Representative text and binary resources are captured with their current encoding, type, and cache headers.

2. Servers or CDNs are assigned a unique priority for Brotli, Gzip, and uncompressed fallbacks for each content type.

3. Requests with different Accept Encoding values ​​check content, headers, cache matches, and end-to-end decoding.

Appropriate content type

• Cache Separation – Caches differentiate representations by accepted encoding and do not deliver an incorrect variant.

Cache Separation

• Bytes transferred per text resource type and delivered content encoding.

• Proportion of compressible responses without expected encoding or with an incorrect cache variant.

Missing Vary Header

• Missing Vary Header – A cache can send a Brotli response to a client that did not request or support it.

• Double Compression – Multiple or unnecessary compression consumes CPU and can increase response size instead of reducing it.

• File Size Only A good compression ratio doesn't fix late discovery or resource blocking.

What to Check Before and After "Using Brotli and Gzip Correctly"

A suitable in-depth resource is available Improving LCP without damaging the visible design"How to Improve LCP Without Breaking the Visible Page Design?"

In addition: Cleanly separating CDN caching and browser caching.

If you want to put "Using Brotli and Gzip Correctly" into practice, you can go to Robust Website Systems Refer back to this. The focus there is on "Resources, Compression and Cache" and "Suitable Content Type."

Conclusion: Using Brotli and Gzip correctly

Brotli and Gzip complement each other as negotiated variants for text resources.

Sources and Further Information

These primary sources make the assumptions, system limitations, and testing methods for "Using Brotli and Gzip correctly" comprehensible.

• RFC 9110: HTTP SemanticsThe HTTP standard defines content encodings, including gzip, and the negotiation of supported methods via Accept-Encoding.

• RFC 7932: Brotli Compressed Data FormatThe IETF standard specifies the Brotli data format and ranks its compression performance against Deflate-based methods.