How Facebook Designs Microinteractions for Feature Discovery article first gist - https://bit.ly/2EtfB6V

facebook-micro-1.scss

// A simple 8 points(coordinates x y) Square 200px by 200px 
// with points equally spread along the corners and middle of each 
// Square side (right model)
clip-path: polygon(
    0 0, 
    100px 0,
    200px 0,
    200px 100px,
    200px 200px,
    100px 200px,
    0 200px,
    0 100px
);