[css-align] [css-multicol] Baseline of multicol container

https://drafts.csswg.org/css-align/#baseline-export

The first baseline set of a multi-column container is generated from the dominant first baseline of the first in-flow line box in the first column box, or is taken from the first in-flow block-level child that contributes a set of first baselines, whichever comes first.

The "first column box" thing here is a bit ambiguous. Is it the first column box in the multicol container, or the first column box in a multicol line/row [1]?

Also, when the spec says:

the first in-flow block-level child that contributes a set of first baselines

Does that apply in any column [2], or does it still apply to the "first column box" only? My guess is the latter.

What about a column spanner, if it has a baseline [3], but the previous column(s) didn't?

[1]

<div style="columns:3;">
  <div style="height:150px;"></div>
  <div style="column-span:all; height:50px;"></div>
  Baseline here? Probably?
</div>

[2]

<div style="columns:3;">
  <div style="height:150px;"></div>
  <div>baseline here? Probably not?</div>
</div>

[3]

<div style="columns:3;">
  <div style="height:150px;"></div>
  <div style="column-span:all;">baseline here? Probably?</div>
</div>

Start with the CSS Align baseline-export section and the CSS Multicol examples in this issue. Determine whether “first column box” refers to the container or a column row, how first baselines are selected across columns, and how column spanners participate; the wording is done when these cases are unambiguous in the specification.