[css-align] definitions of baseline content-alignment seem wrong about which side it adds padding to

[dbaron]

The rules on how to add extra space for baseline content-alignment seem to be duplicated (in slightly different form) between Baseline Content-Alignment and Aligning Boxes by Baseline. However, the former says that padding when aligning to first (last) baseline is added on the start (end) side, and the latter seems to say roughly the same except without specifying that it's padding that's added.

This seems to assume that the align-self of a first (last) baseline aligned element is always start (end). However, I don't see anything that ensures this, and in fact I could imagine it being a useful feature to not ensure this. However, it might also be rather difficult to implement, and I think it needs to be thought through a bit.

For example, in a horizontal flex container, suppose you apply align-content: baseline to two display:block flex items, one of which has align-self: end one has align-self: start. Any padding added for first baseline alignment will certainly be added to the bottom of the first, and the top of the second.

[dbaron]

Hmm, I guess my example isn't valid because of the restrictions on when baseline-content applies (which I thought I'd seen, and then couldn't find when I was writing this issue). But, on the flip side, it's not clear to me why all those restrictions exist. I suppose they do make this a little simpler -- but it's not even clear to me that the complexity of the restrictions themselves is less than the complexity that they avoid. On the other hand, I haven't thought through implementing it.

[fantasai]

As you note, this situation cannot arise because we restrict baseline content-alignment to cases which are compatible with start-alignment (end alignment for last-baseline alignment). Closing as invalid. This only adds a single check to the conditions (to make sure align-self has a good value), and it avoids a massively more complicated baseline-alignment algorithm, that in some cases won't be able to align the baselines at all.

If you would like us to remove the restriction and come up with some definition of how to perform baseline content alignment on an arbitrary collection of start-, end-, and center-aligned boxes, please file a separate issue and Agenda+ it.