Performance · 15 points
Responsive and modern images
One check, stated three ways: what we measure, what would make it wrong, and how to confirm it yourself.
What this measures
Two things per image: whether it offers two or more srcset or <source> candidates, and whether it is served as WebP or AVIF. The result is the weighted share of images doing both.
Where this check can be wrong
A site whose images are generated by a CDN with on-the-fly format negotiation can serve WebP from URLs that end in .png, and this check only reads the markup, so it will under-credit that setup. Small logos do not benefit much from srcset and are still counted.
Verify it without taking our word for it
DevTools → Network → Img filter: check the Type column and the Size column at two viewport widths.