for the uninitiated:
<div style="display:flex;justify-items:center;align-items:center">
<div>I'm centered</div>
<div style="display:flex;justify-items:center;align-items:center">
<div>I'm centered</div>
Back to the drawing board.
<div>I'm centered</div>* I wrote the original without testing but it is justify-content, not justify-items... oops