Web Content Compression Check
The HTML Compression Checker confirms that the HTML response on any URL is being compressed in transit using gzip or Brotli — and reports the compression ratio achieved. Modern browsers all support both algorithms, and enabling compression at the server or CDN typically cuts text-based payloads by 70-80% with no other changes. Sites without HTML compression are silently shipping 4-5x more bytes than necessary.
What This Tool Checks
- Content-Encoding response header
- Compression algorithm used (gzip, br, deflate, zstd)
- Raw vs compressed byte count
- Compression ratio percentage
- Vary: Accept-Encoding header presence
- CDN-level vs origin-level compression
Why It Matters for SEO
Compression is the single highest-ROI server-side performance fix. Enabling Brotli at the CDN cuts HTML, CSS and JS payloads by 70-80%, immediately speeding up Time to First Byte, FCP and LCP. The deploy is usually a single config flag at Cloudflare, Fastly, AWS CloudFront, Nginx or Apache. There is no SEO downside — Google and every other crawler send Accept-Encoding headers and decompress the response.
How to Fix It
Enable Brotli (or at minimum gzip) for all text MIME types: text/html, text/css, application/javascript, application/json, image/svg+xml. At Cloudflare and Fastly this is a single setting. At Nginx use ngx_brotli; at Apache use mod_brotli. Set Vary: Accept-Encoding to preserve cache correctness.
How It Works
We make a request advertising support for gzip, Brotli and zstd via Accept-Encoding, then inspect the Content-Encoding response header and measure the compressed byte count against the decompressed size. The ratio shows whether compression is enabled and how effective it is.
Common Mistakes to Avoid
- Compression enabled at origin but disabled at CDN (CDN serves uncompressed)
- Brotli only enabled at quality 4 (default) when 11 is fine for static content
- Vary: Accept-Encoding missing, breaking caching for compressed responses
- Compressing already-compressed assets (JPG, PNG, MP4) — wastes CPU
- Excluding HTML from the compressed MIME types list
Quick Checklist
- HTML returns Content-Encoding: br or gzip
- Compression ratio above 70%
- Vary: Accept-Encoding present
- CSS and JS also compressed
- CDN compression enabled, not just origin