What is Gzip?

Gzip: the baseline compression every site needs

Gzip is the workhorse of HTTP compression. When a client cannot use Brotli (br), gzip is almost always the answer. Every major browser, CDN, reverse proxy, and search crawler—including Googlebot—understands Content-Encoding: gzip.

For Technical SEO, gzip is not exciting—it is mandatory. Serving uncompressed 200 KB CSS files because someone forgot gzip on in nginx is an unforced error that slows rendering, hurts Core Web Vitals, and burns crawl budget on redundant bytes.

This entry focuses on gzip specifically: how it works, how it differs from Brotli, and how to configure it as the reliable fallback in your compression strategy. For the broader content-encoding negotiation model, see Compression.

How gzip fits the encoding stack

Client Accept-Encoding: br, gzip
         ↓
Server has prebuilt file.br? → send Brotli
         ↓ else
Server gzip enabled? → send gzip
         ↓ else
Send uncompressed (avoid this path)

Gzip is the safety net—not the ceiling. Modern stacks prefer Brotli for static text, gzip for everything else.

Gzip vs Brotli (when to use which)

Policy: enable both; negotiate br first, gzip second.

What gzip compresses well

• HTML — repetitive tags and attributes.
• Minified JavaScript — repeated keywords and punctuation.
• CSS — long selector lists and property blocks.
• JSON — API payloads and JSON-LD in HTML.
• SVG — XML text compresses heavily.
• XML sitemaps — large URL lists shrink dramatically.

A 2 MB sitemap.xml might gzip to 200 KB—faster for crawlers to ingest.

What gzip should not touch

Binary formats with internal compression:

• JPEG, PNG, WebP, AVIF, GIF
• MP4, WebM
• WOFF2 fonts
• PDF, ZIP

Configure MIME-type allowlists—not “compress everything.”

Server configuration examples

nginx

gzip on;
gzip_vary on;
gzip_proxied any;
gzip_comp_level 6;
gzip_types
  text/plain
  text/css
  text/xml
  text/javascript
  application/javascript
  application/json
  application/xml
  image/svg+xml;

gzip_vary on emits Vary: Accept-Encoding—critical for browser caching correctness.

Apache

AddOutputFilterByType DEFLATE text/html text/css application/javascript application/json image/svg+xml

Precompressed static files

Build pipeline outputs app.a1b2c3.js and app.a1b2c3.js.gz. Server maps:

gzip_static on;

Serves .gz with Content-Encoding: gzip when client accepts gzip—zero runtime CPU.

Gzip and dynamic HTML (SSR)

SSR pages generate fresh HTML per request. On-the-fly gzip at level 6 adds modest CPU but saves latency on slow mobile networks—where mobile-first indexing evaluates your site.

For high-traffic HTML, consider:

• Short CDN cache (s-maxage=60) for anonymous pages.
• Edge caching of gzip-compressed variants.
• Edge rendering with compression at PoP.

Headers you should see

Successful gzip response:

HTTP/2 200
Content-Type: text/html; charset=utf-8
Content-Encoding: gzip
Vary: Accept-Encoding

Verify with:

curl -sI -H 'Accept-Encoding: gzip' https://yoursite.com/ | grep -i encoding

Gzip + minification interaction

Minification removes human-readable redundancy; gzip exploits byte-level patterns. Order matters:

1. Minify JS/CSS in build.
2. Gzip (and Brotli) the minified output.
3. Hash filenames for immutable caching.

Skipping minification leaves easy wins on the table—gzip cannot compress verbose comments you should have deleted.

SEO and crawl implications

Faster JavaScript rendering

Smaller JS downloads complete sooner—CSR and hydration finish before crawler render timeouts.

Sitemap and feed efficiency

Large sitemaps gzip cleanly. Ensure your server gzips application/xml responses or serves .xml.gz with correct headers.

Third-party scripts

You cannot gzip assets on other domains. Self-host critical JS or choose vendors that gzip/br their CDNs.

Troubleshooting gzip issues

Migration path to Brotli without dropping gzip

Enable Brotli on CDN/origin, precompute .br alongside .gz in CI, and negotiate br first with gzip fallback. Gzip stays enabled—some proxies still strip br.

How Crawlox helps with gzip-related performance

Crawlox crawl metrics highlight overweight text responses on your origin—often indicating missing gzip/Brotli or unminified assets. While gzip is infrastructure, its absence surfaces as bloated pages in crawl exports, prompting fixes that improve both user experience and crawler throughput before indexation quality suffers.