When Sam says, “that item will automatically extend its specified margin to occupy the extra space in the flex container,” the way my empty filing cabinet brain interprets that is like so:
Setting the margin property on a flex child will push the child away from that direction. Set margin-left to auto, the child will push right. Set margin-top to auto and the child will push to the bottom.
[...]
Why is this useful to know? Well, I think there are a few moments where justify-self or align-self might not get you exactly what you want in a layout where using auto margins gives you that extra flexibility to fine-tune things.
Back in the day we didn't have flex and had to use dark arts to center divs. Young frontend devs these days have it easy. Heck, back in the day there is no frontend devs, just cgi-bin programmers. Get off my lawn!