What you've posted will only center a `<div>` with a known width (ie, for `margin: auto` to center a child element within its parent, the width of the child mus

What you've posted will only center a `<div>` with a known width (ie, for `margin: auto` to center a child element within its parent, the width of the child must also be specified). Not sure if you're mistaken in your comment or there's a bug in the site. When I select all unknown dimensions and a block-level element, it gives me a flexbox output (the right answer, IMO).