Same place you click to build ROM has build web, (you have to compress this "web" folder to a zip file after its complete (to be able to post it to itch))
I am also working on splatterhouse gb!!! Also In Gbs!! (But mine is just a mini game in a huge RPG that lets you play a bunch of different mini games)
Edit*
Love it,
mine is shrank, (yours is 16 tall mine is 8 tall) yours may have issues with more enemie types added (Sprite tile limits)