Gotcha! Duh, I should have asked that first... I've heard of Firefox not liking to run web games correctly but I totally forgot about that aspect. Thank you for letting me know!
And thanks! Yeah, this concept of negative space collision environments where two characters / systems exist in each other's negative space is something I've been obsessed with for some ten years. For white, white is the foreground and black is the background, and for black, black is the foreground and white is the background. I've made several games in this vein:https://itch.io/c/713576/negative-space-games