Built with vanilla JavaScript, the example shows a 15-by-15 interactive crossword puzzle with clickable white cells, blocked black cells, and numbered clue lists for Across and Down entries. The `Crossword` class constructs the grid as a DOM grid of `div` and `input` elements, populating letter positions from a small hardcoded word list. Event listeners handle cell focus and letter input, while the CSS applies the layout and puzzle styling. The implementation uses standard DOM APIs such as `document.createElement` and `addEventListener` to manage interaction without external libraries.