A compressing proxy can reduce the bytes sent across a network by compressing or converting web content—especially text such as HTML, CSS, and JavaScript. It usually cannot make already-compressed images, audio, video, or archives much smaller, and a standard HTTPS tunnel cannot read or change the encrypted page body.
How does a compressing proxy reduce web page size?
Compression works by representing repeated patterns more compactly. Web text often contains redundancy, so HTML, CSS, and JavaScript can be good candidates. MDN gives “up to 70%” as an example of size reduction for some documents; that is not a guaranteed saving for a particular page, proxy, or network transfer. MDN Web Docs: Compression in HTTP
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HTTP content compression is distinct from compression built into a file format and from compression applied only on a particular network link. With HTTP content encoding, the server or intermediary delivers a coded representation that the client can decode. A link-specific mechanism, by contrast, applies between adjacent network nodes and does not necessarily change the representation delivered to the browser.
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Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →How the browser and server choose an encoding
A client advertises the content encodings it can decode in the Accept-Encoding request header. The server selects an appropriate representation and identifies its coding in the response’s Content-Encoding header. Since one URL may have both compressed and uncompressed representations, caches need to account for Accept-Encoding when deciding which response to reuse. MDN Web Docs: Compression in HTTP RFC 9110
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What the proxy may do
A reverse proxy or CDN can compress an uncompressed response, pass through an already-compressed response, or convert between supported encodings. If it changes the response body, it must keep the representation metadata consistent with the new body. Cloudflare documents that some dynamic transformations decompress and then recompress content, even when the origin and visitor use the same encoding. That is a provider-specific behavior, not a rule for every proxy. Cloudflare: Compression
What files cannot be compressed further?
No format is categorically impossible to compress, but a second general-purpose compression pass often has little benefit on files that are already compressed. MDN names images, audio, and video as examples; ZIP archives and JPEG images are other familiar cases. Another pass may leave the size nearly unchanged or even make the file slightly larger. MDN Web Docs: Compression in HTTP
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| Content | Likely result from HTTP compression |
|---|---|
| HTML, CSS, and JavaScript | Often worthwhile because text tends to contain repeated patterns. |
| Images, audio, and video already compressed by their formats | Usually little additional reduction from another general-purpose compression pass; size can sometimes increase. |
| ZIP archives and JPEG images | Already-compressed examples; another pass usually offers little benefit. |
Eligibility also depends on proxy configuration. As one concrete vendor example, Cloudflare documents compression for HTML, CSS, JavaScript, and other text-oriented media types. Its documentation specifies minimum response sizes of 48 bytes for gzip and 50 bytes for Brotli and Zstandard, and says successful responses are compressed only for status 200, while error responses are compressed only for 403 or 404. Those are Cloudflare rules, not universal HTTP requirements. Cloudflare: Compression
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Can a proxy compress HTTPS traffic?
A conventional HTTP proxy commonly uses the CONNECT method to establish an HTTPS tunnel. RFC 9110 describes a tunnel as a “blind relay” that forwards data without changing the messages. Because TLS encrypts the connection to the origin, a proxy operating only as that tunnel cannot ordinarily inspect or rewrite the page body. RFC 9110
An intermediary can process HTTPS content if it terminates TLS and establishes a separate protected connection onward. In that arrangement it becomes an endpoint for the protected connection, changing the trust model; it is not transparent end-to-end compression by an ordinary forward proxy. RFC 9110
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How much data does proxy compression save?
There is no universal saving figure: results depend on the page’s contents, the encodings supported, proxy configuration, and whether the response was compressed already. Text often offers more opportunity than media encoded in already-compressed formats. MDN’s “up to 70%” example describes what some documents may achieve, not a typical or promised result for every page. MDN Web Docs: Compression in HTTP
When evaluating two proxy setups, check which encodings they negotiate, which media types and response sizes qualify, whether each passes through or transforms existing encodings, how HTTPS is handled, and whether caches distinguish encoding variants. Provider-specific documentation matters: one proxy’s thresholds and transformations should not be assumed to apply to another.
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