Why the audit is important
Text compression minimizes the byte size of network responses that include text content. Less bytes downloaded means faster page loads.
How to pass the audit
Lighthouse lists each response that was sent without text compression. Enable text compression on the server(s) that served these responses in order to pass this audit.
How browsers and servers negotiate text compression
When a browser requests a resource, it lists what text compression
encodings it supports in the
accept-encoding request header. All modern
browsers do this. The server encodes the response in one of the formats that
the browser supports and indicates which format it used in the
content-encoding response header.
See Optimizing Encoding and Transfer Size of Text-Based Assets to learn more.
How to enable text compression on your server
Configure your server to compress the response with Brotli, if the browser
supports it. Brotli is a newer compression format, but it's not universally
supported in browsers. Do a search for "how to enable Brotli compression in
<server>" to learn how to implement it, where
<server> is the name of
Use GZIP as a fallback to Brotli. GZIP is universally supported in all modern browsers, but is less efficient than Brotli. See Server Configs for examples.
Check if a response was compressed in Chrome DevTools
To check if a server compressed a response:
- Go to the Network panel in DevTools.
- Click the request that caused the response you're interested in.
- Click the Headers tab.
- Check the
content-headingheader in the Response Headers section.
To compare the compressed and de-compressed sizes of a response:
- Go to the Network panel.
- Enable large request rows. See Use large request rows.
- Look at the Size column for the response you're interested in. The top value is the compressed size. The bottom value is the de-compressed size.
How the audit is implemented
This section explains how this audit is implemented, so that you can understand how the audit's score is calculated.
Lighthouse gathers all responses that:
- Have text-based resource types.
- Do not include a
content-encodingheader set to
Lighthouse then compresses each of these with GZIP to compute the potential savings.