Contact emails:
[email protected], cc: [email protected]

Explainer
N/A

Specification
Changes have not been made to the spec yet, but resolutions around this can be 
found here: https://github.com/w3c/csswg-drafts/issues/14052

Summary
It was resolved in the CSSWG that we should introduce the flow-start/end 
keywords as value aliases for the existing flex-start/end keywords. 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<https://github.com/w3c/csswg-drafts/issues/12803>).  
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.  This could introduce 
compatibility concerns for sites that rely on the existing serialization of 
flex-start/end.  Because of this, we plan to use a second flag to introduce a 
separate gate for the new serialization behavior of flex-start/end so that if 
we receive reports of site breakages as a result, we can disable the 
serialization behavior separately from the new keywords.

Blink component
Blink>CSS<https://issues.chromium.org/issues?q=customfield1222907:%22Blink%3ECSS%22>

Web Feature ID
https://github.com/web-platform-dx/web-features/issues/4282<https://webstatus.dev/features/https://github.com/web-platform-dx/web-features/issues/4282>

Motivation
The start/end alignment keywords follow the physical start/end directions of a 
given container. In layout modes that support reversals, these keywords may 
align content in the opposite direction than authors are looking for. The 
flex-start/end keywords currently allow authors to choose the logical start/end 
of a flex container, but with new layout modes like grid lanes supporting 
reversals, as well, the need for a new more generic way of choosing the logical 
start/end that wasn't flexbox specific was introduced. These new keywords 
provide that solution, and are generic enough to apply to any new layout modes 
or logical impacting adjustments in the future.

Initial public proposal
https://github.com/w3c/csswg-drafts/issues/14052

Requires code in //chrome?
False

Tracking bug
https://issues.chromium.org/issues/555299635

Link to entry on the Chrome Platform Status
https://chromestatus.com/feature/5069051687731200?gate=6279206035128320


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