Reduce proxy bandwidth first by resizing, converting, compressing, and caching images for their actual display size. Optimize CSS with minification and carefully verified removal of unused rules; suppress or stub styles only when you have tested that the page remains usable. Image transformations usually offer the clearest byte savings, while CSS stubbing and URL rewriting carry greater visual and compatibility risks. Measure representative pages before and after changes: there is no reliable universal savings percentage.
What image and CSS stubbing can—and cannot—save
“Stubbing” means replacing or suppressing a resource rather than passing it through unchanged. For images, a proxy can return a smaller, compressed version suited to the space where it will appear. For CSS, it might omit a stylesheet or selected rules, or replace a resource with a minimal response. Those changes can reduce transferred bytes and requests, but they do not have equal risk: an image can often be reduced without changing page structure, while CSS controls layout, readability, and the presentation of interactive states.
Do not treat old figures as a forecast for a current site. A Chromium Blog description from 2013 said images accounted for over 60% of transferred bytes on an average web page; that is a historical description of proxy traffic, not a universal current measurement. A 1997 W3C HTTP performance test reported up to 9,200 bytes saved in its revalidation test by using CSS1 to reduce embedded objects, approximately 30% total bandwidth savings in that test, and about 35% in a separate combined HTTP/1.1, transport-compression, CSS, and PNG scenario. The page, protocols, and test conditions were specific to that report. Measure your own audience and workload instead of applying those percentages to a modern site.
Measure a baseline before changing responses
Choose representative pages, devices, routes, and network conditions. A homepage alone may not represent an image-heavy product listing, article page, or authenticated application. Record both resource transfer and user-visible behavior so that a bandwidth reduction is not counted as a win if it breaks the page.
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- Bytes by type: image, CSS, JavaScript, HTML, and fonts. Record transferred bytes, not just uncompressed file sizes.
- Request count: total requests and requests by type, including those initiated after initial HTML parsing.
- Loading experience: First Contentful Paint (FCP) and Largest Contentful Paint (LCP), under repeatable conditions.
- Functional checks: navigation, forms, menus, responsive layouts, focus visibility, and other important interactions.
- Proxy behavior: cache hit rate, transformation time, origin traffic, and errors or fallbacks.
Keep a page-level sample before and after each policy change. Change one class of resources at a time; otherwise, you will not know whether a byte reduction or a regression came from image processing, CSS changes, or caching.
Resize and convert images at the proxy or edge
Serve the size the page actually displays
An image downloaded at several thousand pixels wide for a small card wastes transfer capacity even if the file is already compressed. Select output dimensions based on the rendered slot and the device’s pixel density. Preserve enough resolution for the largest intended display, but avoid returning the original dimensions to every client by default. Use known layout sizes or a controlled set of width variants rather than allowing arbitrary query values to generate unlimited distinct outputs.
Choose an efficient format and compression level
Convert an image to a suitable output format when the client and image content support it. The right format and quality depend on whether the asset is photographic, graphic, transparent, or otherwise sensitive to artifacts. Negotiate format using reliable request information, and include the selected output format in the cache key. Do not assume every upstream image can safely become the same output type.
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Cache the transformed response
Transformations consume CPU as well as bandwidth. Cloudflare’s Workers Images binding accepts image bytes, supports chained transformations, and allows output-format selection; its documentation warns that transformed responses are not automatically cached. If repeated requests are not cached, the source may be decoded and re-encoded on every request. Configure cache behavior and response Cache-Control headers deliberately.
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imgproxy supports on-demand resizing, processing, conversion, and compression, and is designed to sit behind a CDN or reverse proxy. Its cache guidance recommends avoiding duplicate CDN image optimization. In either architecture, put the transformation behind a cache where practical and make sure the cache key accounts for every output-affecting input, including dimensions and format, plus relevant client hints. Otherwise, clients can receive a variant generated for different parameters, or the cache can accumulate needless variants.
Do not stack image optimizers blindly
Applying two independent image optimizers in sequence can add processing latency and complicate cache behavior without reliably improving the result. Decide which layer owns each transformation. If an upstream CDN already resizes or converts images, do not repeat the same work at the proxy unless you have measured a specific benefit and verified the resulting cache keys.
Optimize CSS without breaking the page
Start with transformations that preserve meaning
Minify stylesheets to remove avoidable transfer overhead. Remove rules only when they are proven unused for the relevant route or page state; a rule that appears unused on initial load may be needed after interaction, at a different viewport, or after client-side navigation. Splitting genuinely route-specific CSS can reduce bytes for users who do not visit other routes, but it can add requests or loading coordination. Check the net result rather than assuming fewer bytes in one file means a faster page.
Where possible, replace unnecessary plain-CSS @import chains with link-based stylesheet loading. CSS is render-blocking, and chained imports can delay discovery and application of styles. Keep critical layout and readable text styling available early rather than trying to save bytes by withholding all styles.
Stub only noncritical styles, with a fallback
CSS background-image resources are discovered later than ordinary markup images by the preload scanner. Deferring or suppressing them can reduce secondary transfers, but may leave backgrounds or other visual details missing. If you selectively stub styles, preserve rules that support:
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- Layout, spacing, and responsive breakpoints needed to keep content readable.
- Typography and contrast needed for legibility.
- Visible focus indicators and hover or active states that communicate interaction.
- Visibility and state changes for menus, dialogs, and other controls.
Prefer an allowlist or a route-aware policy over broad deletion. Test multiple viewport sizes and interaction states, and monitor visual regressions after deployment. If a policy cannot reliably identify a noncritical style, pass the original through.
Rewrite resource URLs only when the proxy can parse them
Rewriting URLs through a proxy can route resources through a transformation or cache layer, but a URL can appear in different kinds of content. Apache mod_proxy_html rewrites matching URLs in HTML; its documentation says links in JavaScript and CSS are ignored unless extended handling is used. Inline scripts and stylesheets are buffered for parsing, which makes parser limits and content handling relevant to reliability.
Use content-type detection and a parser appropriate to the resource rather than replacing strings indiscriminately. Keep limits on how much content must be buffered. If parsing or rewriting fails, serve the original response instead of emitting a partial document. A fallback preserves availability even when the optimization cannot be applied.
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Choose a policy by savings, risk, and operating cost
| Approach | Likely benefit | Main risk or cost | Best fit |
|---|---|---|---|
| Resize, convert, and compress images | Directly reduces image bytes when the original is larger or less efficient than the display requires. | Transformation CPU, cache variants, and possible quality or compatibility issues. | Image-heavy traffic where dimensions and output formats can be controlled. |
| Minify CSS and remove verified unused rules | Reduces stylesheet transfer size while retaining required styling. | Incorrect unused-rule detection can break less common routes or interaction states. | Sites with a repeatable build or route-level test process. |
| Suppress noncritical CSS or background images | Can avoid selected transfers. | Higher risk of visual incompleteness or broken presentation. | Constrained views where the omitted styles are known to be nonessential. |
| Rewrite URLs in proxied content | Can steer matching resources through an optimization path. | Syntax differences across HTML, CSS, and JavaScript; buffering and parser failure modes. | Gateway deployments with content-aware parsing and a pass-through fallback. |
Compare measured bytes saved with visual fidelity, cacheability, cache-key complexity, transformation CPU, latency, correctness across resource references, and failure behavior. A transformation that saves bytes but multiplies cache variants or delays responses may not improve the overall service.
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- Establish representative baselines. Save resource-level measurements and functional checks for important routes, devices, and viewport sizes.
- Start with image variants. Add display-appropriate dimensions, format selection, compression, and a cache policy. Verify cache keys and response headers before widening traffic.
- Validate the transformed output. Check visual quality, transparency where relevant, browser compatibility, and the behavior of repeated requests.
- Optimize CSS conservatively. Minify first, then remove only rules shown to be unused across route and interaction states. Test responsive layouts, forms, navigation, and focus states.
- Canary and compare. Expose a limited share of traffic to the policy and compare bytes, FCP/LCP, errors, cache hits, and functional results against the baseline.
- Keep a pass-through path. Disable a transformation or serve the original resource when parsing, conversion, or validation fails.
Troubleshoot common failures
- Image output is stale or has the wrong size: Check that width, height, output format, and relevant client hints are included in the cache key. Purge or version cached variants after changing transformation behavior.
- Repeated requests use too much CPU: Confirm transformed responses are actually cached and that freshness headers allow reuse. Check whether an upstream CDN and proxy are both optimizing the same image.
- Images appear soft, distorted, or unexpectedly large: Compare the rendered slot and device pixel density with the output dimensions; inspect format negotiation and compression settings. Avoid one fixed size for every context.
- Page layout breaks after CSS removal: Restore the affected rules and test alternate routes, viewport breakpoints, and post-interaction states. Narrow the removal policy rather than suppressing a whole stylesheet.
- Backgrounds or icons disappear: Check CSS background references and whether they are deferred or omitted by the policy. Restore those resources where they carry important content or state information.
- Rewritten pages contain broken links or partial markup: Verify the response content type and parser coverage. JavaScript and CSS references may not be handled by an HTML-only rewriting pass; on parse failure, serve the original resource.
- Transfer size falls but the page feels slower: Review transformation latency, cache misses, request count, and render-blocking CSS. A smaller response can still arrive later or postpone rendering.
Or skip the browser setup
If your goal is to capture a clean website screenshot rather than optimize a general-purpose proxy, ScreenshotNeo provides a one-request screenshot API. This is a screenshot alternative, not a substitute for proxy-side resizing, CSS optimization, or transformed-response caching. A cURL request looks like this; see the ScreenshotNeo API documentation for configuration details:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpEquivalent Python and Node.js requests are:
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);ScreenshotNeo can accept cookie or consent banners as a visitor and remove more than 60 known consent platforms, newsletter popups, and chat widgets before capture; each of those steps can be turned off. Bot checks, blank pages, failed loads, and cache hits are not billed, and responses identify page verdict and billing status in headers. Its MCP server provides screenshot tools for AI agents. The free plan includes 1,000 screenshots a month with no card; paid plans start at $5 for 3,000. Sign up for 1,000 free screenshots a month, with no card required.
Frequently Asked Questions
Should a proxy remove every image that is not immediately visible?
No. An image that is off-screen initially may become visible during scrolling or interaction, and some images carry meaningful content. Prefer delivering a smaller appropriate variant or deferring a known noncritical asset over removing images indiscriminately.
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No. Savings depend on the target pages, traffic mix, existing compression and caching, and transformation policy. The historical figures cited in the article do not establish a current general-purpose guarantee.
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