For educational purposes, you may read the source code of this program. If you want to do anything other than read it, and execute it on this web site, ask me for permission.

There are three top-level pages, index.php, intermediate.php, and expert.php. Each sets PHP globals specifying the board layout, then invokes the server-side script minesweeper.php. This creates the actual HTML, including a table containing the requisite number of squares, and a call initializing the browser-side script minesweeper.js. The HTML uses a stylesheet, minesweeper.css.

The graphics are achieved by the table layout and a couple of uncommon Unicode characters. The only actual images are the smileys:

     

There's also the source-listing page lister.php, and this page source.php.