sgt-puzzles/html/dominosa.html

Dominosa
<p>
Tile the rectangle with dominoes (1&times;2 rectangles) so that
every possible domino appears exactly once (that is, every possible
pair of numbers, including doubles).
<p>
Click between two adjacent numbers to place or remove a domino.
Right-click to place a line between numbers if you think a domino
definitely cannot go there. Dominoes light up red if two identical
ones appear on the grid.