Why it looks like this

Netscape 4 and Internet Explorer 4 had no DOM. There was no innerHTML, no createElement, no way to change a page after it had loaded. Almost nothing on a rendered page was mutable.

One thing was: the src of an already-placed <img>. Assigning to it swapped the picture in place. That single writable property is the entire rendering engine of this game — the board is a table of images, and every move is a picture swap.

Two more things follow. document.write destroys the document it writes into, taking every variable and function with it, so the engine had to live in a frame it never wrote to: a strip twenty pixels tall, holding all the code and all the state, driving the board and the counters from outside. And there was no dependable right-click — oncontextmenu did not exist yet — so every cell was drawn as two images. Click the left half to uncover; click the right half to flag.

“Since at the time of creating the application there was no solid way to detect right-clicks, the right click can be done by clicking the right side of the cells.” — the original launcher page, 1999

The nice part is that the tile teaches you. Hovering either half repaints both: the side you are over lights up and the other dims, so a covered cell splits into a labelled pair under the cursor and explains its own controls. That is a real piece of interaction design working around a missing event, and it is worth keeping.

So the rebuild keeps it — as a mode. Switch to 1999 and the cells split again, the flood fill stops at diagonals the way the original's did, and your first click can kill you. Switch to Today and you get a real right-click, long-press on a phone, chording, and a first click that is always safe. The differences are listed on the game page.