position-items: center
This declaration would position all grid items in the center, including items in the last row.
I'm aware it would be tricky to implement this, especially with some combination of numbers of items, but still, it would be nice to have.
In the example below, I've added a border to these items.
See other not-so-valid CSS things
collapsing: none
collapsing: none is an invalid CSS declaration I wish existed.
place: center
place: center is an invalid CSS declaration I wish existed.
font-family: system-font
font-family: system-font is an invalid CSS declaration I wish existed.
position-items: center
position-items: center is an invalid CSS declaration I wish existed.
position: temporal
position: temporal is an invalid CSS declaration I wish existed.
color: anthracite
color: anthracite is an invalid CSS declaration I wish existed.
float: top
float: top is an invalid CSS declaration I wish existed.