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modern-format

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Use when reviewing image assets, markup, and CDN or build transforms related to Use modern image formats (WebP, AVIF). Check encoded size, rendered size, loading strategy, and above-the-fold impact together.

skills/modern-format/SKILL.md

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# Use modern image formats (WebP, AVIF)

Images are typically 50%+ of page weight. Switching from JPEG to WebP alone reduces transfer sizes by 25-35% with no visible quality change, directly improving Largest Contentful Paint (LCP) and saving bandwidth for users on mobile data plans. These savings compound across every visitor and every image on the site.

## Quick Reference

- WebP is ~25-35% smaller than JPEG at equivalent quality (per web.dev)
- AVIF is ~40-50% smaller than JPEG at equivalent quality (per web.dev)
- Use `<picture>` with `<source type="image/avif">` and `<source type="image/webp">` plus an `<img>` fallback
- WebP has 97%+ global browser support; AVIF is supported in all modern browsers as of 2023

## Check

Scan all <img> elements and CSS background-image declarations in this codebase. Identify: 1) Any <img src> pointing to .jpg, .jpeg, or .png files (not served via a CDN that auto-negotiates format). 2) Any <picture> elements that lack <source type="image/webp"> or <source type="image/avif"> entries. 3) Any CSS background images using JPEG or PNG when a WebP equivalent could be used. Report each finding with file path and line number.