Performance · 15 points
Image dimensions
One check, stated three ways: what we measure, what would make it wrong, and how to confirm it yourself.
What this measures
The share of <img> elements that carry both a width and a height attribute, which is what lets the browser reserve the box before the image arrives.
Where this check can be wrong
Images sized purely in CSS with an explicit aspect-ratio reserve the box too, and this check cannot see that, so it under-credits those pages. Decorative images that never shift layout are counted the same as large hero art.
Verify it without taking our word for it
View source and search for <img: every one should show width="…" height="…" next to its src.