Some of them are harder to spot than others, and sometimes when I get into a groove noticing "three of these have a black square but one does not" it takes me a while to notice "three of these have an outer square and one has an outer circle." But there are ultimately only 40 sets and they all have an odd one out! I can provide the answer to any set you screenshot (or right-click-save-as on the game).
It's all in HTML and JavaScript. I make all of my games in HTML5 Canvas these days.
Thanks for playing and commenting!