People implement Doom in CSS. It's clearly more than just some style notes for the browser.

I think you may be confusing that with something else. You can't create a complex game with just CSS. I'd love to be wrong but provide a link.

The first one looks like a big part of the functionality is from the JavaScript file. It's crashing my iPhone so I'm having trouble digging into it too much. The second link, however, does seem to be done in pure css. Impressive! Thank you for the links. I stand corrected.