WPT details: https://wpt.fyi/results/css/css-scroll-snap
This feature lets a single-axis scroll container (for example, one with overflow: auto clip) handle scroll snap area propagation and alignment separately for each axis. Single-axis scroll containers are now supported (https://chromestatus.com/feature/5067363861004288). Without this feature, such a container captures a snap area on both axes, even the axis it can't scroll. The snap area then can't align on that axis. With this feature, if a snap area sets alignment on both axes, the single-axis container handles only the axis it scrolls. The other axis goes up to the nearest ancestor scroll container that scrolls in that direction.
This is part of the single-axis scroll container feature (https://github.com/explainers-by-googlers/single-axis-scroll-containers) to align the scroll snap behavior with the sticky header in a single-axis scroll container.
Explainers: https://github.com/explainers-by-googlers/single-axis-scroll-snap