The HTML comes in three top-level pages (index.php, intermediate.php, and expert.php). From the browser’s view, each of these is static HTML, but on the server they each used a script (minesweeper.php) to construct the playing area.
The game play is implemented entirely in your browser, by the client-side script (minesweeper.js).
The page layout is controlled by a style sheet (minesweeper.css). The graphics use some Unicode characters (“∞”, “☀”, and “×”) and the smiley images: