According to: https://www.w3.org/TR/css-flexbox-1/#flex-wrap-property

The flex-wrap property controls whether the flex container is single-line or multi-line, and the direction of the cross-axis, which determines the direction new lines are stacked in.

So if you set the "flex-wrap" property to "wrap", the flex container will be a multi-line container, and then "align-content" works.