This is something that I have been working on for some time now. It's called Mariohub and is supposed to be a platform to share Mario hacks for the NES and SNES Mario games.
The current version supports playing individual levels (which are 1-1 in both SMB and SMB3), as well as uploading them. Once there are enough levels uploaded, I will include the Marathon mode, where you get a set of random levels in a difficulty range (various parameters will be up to the user, like the clear rate, number of levels, number of lifes...).
In order to play and upload the levels, you need to provide the path to your fceux executable and the roms for SMB and SMB3 in the settings.
However, I do not have a windows system right now, which is, why I can only provide the source code and a Linux executable, so it would be amazing, if anyone with electron on their Windows system could provide a build.
The source code is here
Any feedback is greatly appreciated.
SNES support is added most likely as soon as lua support on bizhawk is available for all operating systems.