An ambitious forum user at AtariAge has uploaded a playable Atari 2600 demake
of World 1-1 from the original Super Mario Bros
. The demo includes the entire level with warp pipes, yellow bonus blocks and flagpole.
Developer Chris Spry, who posts as "Sprybug" on the site, hopes to add another 15 levels, up to World 4-4. He'd also like to add a Koopa Kid to the end of each world. It may not be canon, but we aren't complaining.