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responsive-size

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Use when reviewing image assets, markup, and CDN or build transforms related to Serve images at the correct display size. Check encoded size, rendered size, loading strategy, and above-the-fold impact together.

skills/responsive-size/SKILL.md

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# Serve images at the correct display size

A single oversized hero image can add several megabytes to a page load unnecessarily. On mobile devices with 375px screens, serving a 1600px image downloads 4x the data users need. The 'Properly size images' Lighthouse audit consistently identifies this as one of the highest-impact performance opportunities on most websites.

## Quick Reference

- Serving a 1600px image in a 400px container wastes ~16x the necessary bandwidth
- Use `srcset` with width descriptors and `sizes` with actual layout widths
- The `sizes` attribute is critical—without it the browser assumes `100vw` and picks the largest image
- Lighthouse 'Properly size images' audit reports estimated savings per oversized image

## Check

Audit images in this codebase for oversizing. Use Lighthouse 'Properly size images' audit results as the primary signal. For each <img> in the HTML: 1) Compare the image file's intrinsic width to the element's rendered CSS width (check CSS rules, container constraints). 2) Flag any image where the intrinsic width is more than 2x the rendered width at any common viewport size. 3) Check for srcset usage—images over 200px wide should have srcset variants. 4) Check sizes attribute accuracy—does it reflect the actual CSS layout?
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