`align-content` CSS property works for block containers and table cells. The property worked only for flex containers and grid containers. It supports block containers and table cells, that is to say `display: block`, `display: list-item`, `display: table-cell`, etc.
If web authors wanted to center the content vertically, they would use some weird hacks or change the display type to flex/grid. They can easily center the block container using the align-content property.