WPT details: https://wpt.fyi/results/css/css-align?label=master&label=experimental&aligned&q=css-align
Introduce the flow-start/end keywords as value aliases for the existing flex-start/end keywords as resolved in the CSSWG. These are needed for alignment in the case of placement reversals. flow-start in CSS Grid-Lanes would indicate the starting edge of the container at which placement begins, which is useful in the case where placement is reversed (`fill-reverse`, `track-reverse`, discussed in this CSSWG issue: [css-grid-3][masonry] item-flow row vs. column in masonry layouts · Issue #12803 · w3c/csswg-drafts). Because these are acting as value aliases, we expect flex-start/end to follow the logical directions in Grid Lanes, as well, which will expand the definition of these keywords from only applying special behavior in flexbox to applying to any layout mode whose logical start/end is different from its physical start/end. For serialization, if either flex-start/end or flow-start/end is used, the returned serialized value should be flow-start/end.
In grid-lanes, there is currently no alignment keyword to allow us to align based on the start of content placement when reversal is present; instead, we can only currently use the logical container start/end. We wanted a keyword that behaves the way flex-start/end does in flexbox containers. Introducing flow-start/end allows us to align based on content placement start/end, and aliasing it to flex-start/end means that we can have the same keyword used across flexbox, grid, and grid-lanes layout with the same expected behavior.