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Guidelines · WSG 4.2

Optimize caching and support offline access

A returning visitor should download almost nothing. Missing cache headers make every visit as expensive as the first, which is invisible in a single measurement but doubles real-world cost.

How to satisfy it

  • Serve fingerprinted static assets with a long max-age and immutable.
  • Use short max-age with stale-while-revalidate for HTML that changes.
  • Never send no-store on assets that are safe to reuse.
  • Verify with a second load: repeat-visit weight should be a fraction of the first.

Measured pages failing this guideline

From the 60 most recently measured pages. Each links to its full report.

This page explains how Carbonless applies WSG 4.2. The guideline itself is published by the W3C Sustainable Web Design Community Group: Web Sustainability Guidelines.