Nice game!

This made me uncomfortable though (about CSS grid, not about the game):

grid-area: row start / column start / row end / column end;

So you have to put the rows (Y axis coordinates) first and columns (X axis coordinates) second, i.e. the opposite of how it's done in every other situation - i.e. draw_rect(start_x, start_y, end_x, end_y)

(1, 1, 3, 4) in every other language would draw a box 2 wide and 3 high, but in css grid it selects an area 3 wide and 2 high.

Also the fact it uses 'row' and 'column' to describe the gridlines rather than the actual rows and columns irked me.

I'm sure I'll get over it!