cqw

1cqw is a hundredth of the width of whichever ancestor is the nearest query container. Giving an element container-type: inline-size makes it one, and descendants can then size themselves from it. A card whose heading is font-size: clamp(1.25rem, 6cqw, 3rem) adapts to the slot it was placed in and ignores the window, which is the reason the unit exists: in a container 400px wide the heading is 24px, in one 1000px wide it stops at 48px, and the size of the window changes neither.

The width is that of the container’s content box. A container 400px wide with 50px of padding on each side (box-sizing: border-box) offers 300px, so 100cqw inside it is 300px, not 400px. Padding is not counted, and a card that looks 400px wide to the eye measures less.

With no query container among its ancestors, the unit reads the small viewport: in a 1280px window, 10cqw with no container-type above it came to 128px, the same as 10svw. The console says nothing and the card still renders, so the symptom is a component that follows the window instead of its slot. A second container in between gives wrong numbers of another kind: the nearest ancestor with container-type answers, whether or not it is the one you had in mind, and an ancestor without the property is passed over.