A pentomino is a shape made of five unit squares joined edge-to-edge. There are exactly 12 of them, named after the letters they resemble: F, I, L, N, P, T, U, V, W, X, Y and Z. Each one is drawn here on its own canvas, scaled to fill the canvas.
Each pentomino has its own colour; the dark outline traces the whole shape, and the thin grey lines show how it splits into its five squares.
The outer (pentomino) boundary uses the chosen line width; the inner square divisions use half that width in grey (#888). Each shape is centred and scaled so it reaches the canvas edges, leaving a couple of pixels so the outline is not clipped.