FWIW the equivalent code with this API is:
.BlueSquare {
width: 100px;
height: 100px;
top: 0px;
left: 0px;
anchor-name: --BlueSquare;
}
.RedSquare {
width: 100px;
height: 100px;
top: calc(anchor(--BlueSquare bottom) + 10px);
left: calc(anchor(--BlueSquare right) + 10px);
}