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Guidelines · WSG 2.9

Optimize media to reduce resource use

Media is usually the largest share of a page by weight, so it is where transfer, and therefore the estimate, moves the most. A single unoptimized hero image can outweigh every script on the page.

How to satisfy it

  • Serve AVIF, falling back to WebP. Both typically halve JPEG weight at the same visual quality.
  • Use responsive srcset sizes so a phone never downloads desktop pixels.
  • Set width and height (or aspect-ratio) so images do not force layout work while loading.
  • Lazy-load anything below the fold, and never autoplay video that a visitor did not ask for.
  • Replace animated GIFs with muted looping video: the same clip is typically a tenth the size.

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 2.9. The guideline itself is published by the W3C Sustainable Web Design Community Group: Web Sustainability Guidelines.