It's an unofficial fan made project, but oh my god it's the tits! You play SMB, but it's got all these quality of life improvements.
Like the background and clouds aren't static like they would be in the NES. There's options to backtrack. You can change the clock to unlimited, or change the seconds to real seconds. You can change how damage is done. American style, or Japanese style.
They included Super Mario Bros Special. Which is a rare 1986 Super Mario game, which was released by Hudson Soft. In the 1986 version the screens did not scroll with you. Instead you move forward, and the screen transitions. Well this game takes those levels, and lets you play them in the original SMB engine, with SMB graphics.
They included a mode from SMB DX where you had to hunt for 5 red coins, a yoshi egg, and get a certain score.
They included a boo race mode.
And Super Mario Bros 2 (Japan)
I don't even know what All Night Nippon Super Mario Bros is.
They included resource packs, which change the graphics. They have a stage builder. You can share or download others custom levels.
It has achievements.
Guys, this game is the real deal.
I have, and it's excellent! I spent a lot of time playing it on my Steam Deck when it came out. There is also a Super Mario World Remastered by the same dev, but it was abandoned. It's not up to the same level of quality, but it's definitely the best way to play multiplayer Mario World regardless!