Single click on a square to select it, click again on the square to turn it black.
Use the arrow keys to navigate.
Pressing Backspace removes the letter in a space, and moves backward, Delete does the same, but moves forward.
Pressing Enter uses the API of Data Muse to generate the most frequently used word that fits the requirements of the highlighted section.
Please let me know about any bugs you find—this is a work in progress.