This game extension is ridiculously small, at only 65 kB in size. The interface can be configured to show guiding lines and block highlight (default), light guiding lines, or only the focused cell. The numbers may be input by clicking on the cell and selecting the number from a pad that pops up or by typing the corresponding number on the main keyboard or on the keypad (numeric lock doesn't even need to be activated!). The number is input in the focused cell, either by moving to it with the cursor keys or by hovering with the mouse, thus the game may be played using keyboard only, mouse only or both at the same time. To erase a number, just type zero. The only problem I noticed is that, upon opening the game bubble, it's necessary to click on any part of it to give it focus (for example, on its lower bar, there's plenty of space there to click without messing with the game board) or hitting the key, so the focus is transferred to the game and the keyboard may be used, otherwise, Firefox keeps the focus on itself.