If the goal is to create a grid with equal height rows, where the tallest cell in the grid sets the height for all rows, here's a quick and simple solution:
grid-auto-rows: 1frGrid Layout provides a unit for establishing flexible lengths in a grid container. This is the fr unit. It is designed to distribute free space in the container and is somewhat analogous to the flex-grow property in flexbox.
If you set all rows in a grid container to 1fr, let's say like this:
grid-auto-rows: 1fr;
... then all rows will be equal height.
It doesn't really make sense off-the-bat because fr is supposed to distribute free space. And if several rows have content with different heights, then when the space is distributed, some rows would be proportionally smaller and taller.
Except, buried deep in the grid spec is this little nugget:
7.2.3. Flexible Lengths: the
frunit...
When the available space is infinite (which happens when the grid container’s width or height is indefinite), flex-sized (
fr) grid tracks are sized to their contents while retaining their respective proportions.The used size of each flex-sized grid track is computed by determining the
max-contentsize of each flex-sized grid track and dividing that size by the respective flex factor to determine a “hypothetical1frsize”.The maximum of those is used as the resolved
1frlength (the flex fraction), which is then multiplied by each grid track’s flex factor to determine its final size.
So, if I'm reading this correctly, when dealing with a dynamically-sized grid (e.g., the height is indefinite), grid tracks (rows, in this case) are sized to their contents.
The height of each row is determined by the tallest (max-content) grid item.
The maximum height of those rows becomes the length of 1fr.
That's how 1fr creates equal height rows in a grid container.
As noted in the question, equal height rows are not possible with flexbox.
Flex items can be equal height on the same row, but not across multiple rows.
This behavior is defined in the flexbox spec:
In a multi-line flex container, the cross size of each line is the minimum size necessary to contain the flex items on the line.
In other words, when there are multiple lines in a row-based flex container, the height of each line (the "cross size") is the minimum height necessary to contain the flex items on the line.