Quick answer
Enable a suitable compression method for compressible text responses when it improves transfer efficiency. Brotli and gzip are common choices, but the right configuration depends on the server, proxy, content type, and client negotiation. NOTABIS flags an HTML response when no Content-Encoding header is observed; it does not benchmark every asset or prove a user-visible speed problem.
What the issue means
A client can advertise supported encodings in Accept-Encoding, and the server may respond with compressed bytes and a Content-Encoding header. Typical examples are br for Brotli and gzip for gzip. Images, video, and already-compressed archives often do not benefit in the same way.
Why it matters for SEO
Smaller HTML responses can reduce transfer work, especially on slower connections. Compression is one part of delivery performance; it does not fix slow server processing, excessive JavaScript, poor caching, or large images. Search visibility is not guaranteed by adding a header.
How NOTABIS detects it
The compression-review rule checks the measured HTML response for a Content-Encoding header. If no encoding is advertised, NOTABIS records a low-severity review issue. It does not calculate compression ratio, verify that bytes are actually compressed correctly, or inspect every resource type.
How to check it yourself
Compare headers with a request that advertises compression:
curl -sSI -H 'Accept-Encoding: br, gzip' https://example.com/
Look for Content-Encoding, Vary: Accept-Encoding, and a response that is not already compressed by another layer. Test through the real CDN or proxy, not only the origin server.
How to fix it
Configure compression at the layer that serves the response, commonly the web server, CDN, or reverse proxy. Compress HTML, CSS, JavaScript, JSON, and SVG where appropriate. Exclude formats that are already compressed, and ensure caches vary correctly when they store compressed and uncompressed variants.
After changing configuration, test multiple content types, cache hits and misses, and older clients supported by your product.
When it is intentional or does not need fixing
A response may be tiny, already compressed elsewhere, or served through a platform whose headers are controlled outside the application. Private or unusual responses may have different priorities. Do not enable compression blindly for every binary format.
Related issues
Review slow server responses, large HTML responses, and cache policy together.
Run a NOTABIS audit
NOTABIS shows whether compression was advertised on measured HTML responses so you can identify origins or delivery paths that need a closer performance review.