will be slightly different for each browser and the last files can change up a bit
if you are using chrome then they are in the chrome user "appdata" area
eg for example if I run my game in chrome with the construct 3 net preview
then the local storage is
you can ppen with text and see the json
edit - update
i was just messing around with saving and seeing what happens in that file area and there is enough weird stuff going on with different files and folders there that I'm sure that mortal men are not supposed to see or touch anything there. i would say just stick to read write to it through the game.