The section 11.6 of the CSS Grid Layout specification has some inconsistencies, broken links and obscure statements that I think they deserve some clarification.
https://drafts.csswg.org/css-grid/#grid-baselines

Otherwise, If the item has no alignment baseline in that axis, then one is first synthesized from its border edges.

• What axis ? I think that text comes from the version which allowed block-axis baselines.

Otherwise, the grid container has no first (last) baseline set, and one is synthesized if needed according to the rules of its alignment context.

• The link 'alignment context' is broken.
• What does it means "synthesized according to the rules of its alignment context." ?
• Why "if needed" ?

From the last comment in #439 I came to the conclusion that when the grid container has no baseline set it should use its border bottom edge.