Responsive variations also exist for align-items. Choose from start, end, center, baseline, or stretch (browser default).

Use align-items utilities on flexbox containers to change the alignment of flex items on the cross axis (the y-axis to start, x-axis if flex-direction: column). Responsive variations also exist for justify-content. Choose from start (browser default), end, center, between, or around. Use justify-content utilities on flexbox containers to change the alignment of flex items on the main axis (the x-axis to start, y-axis if flex-direction: column). Responsive variations also exist for flex-direction. It can have any of the following values: flex-start, flex-end, center, baseline, and stretch.Flex item 1 Flex item 2 Flex item 3 Flex item 1 Flex item 2 Flex item 3 The align-items property determines how flex items are positioned within a flex line, along the cross-axis.

align-items positions flex items within a flex line, along the cross-axis In other words, unless you have more than one flex line, you do not need the align-content property. The align-content property is relevant only if your flex-container is set to wrap. If your flex-direction is set to column, your cross-axis is the horizontal axis. If your flex-direction is set to row (which is the default), your cross-axis is the vertical axis. Remember that your cross-axis depends on your flex-direction. Both properties function along the cross-axis.

I think this similarity contributed to my initial confusion. The effect of both align-items and align-content is visible along the cross-axis. Comparison of align-items and align-content Both function on the same axis

Display flex vertical align