Based on the description of the canvas tag ...
...it should be used strictly for HTML5 games using the canvas element. However a lot of questions about Unity also use the same tag, because the drawing surface for unity projects is also called like that. Same for games using the Canvas object in the
java.awt package (altough, they come up less frequently).
I think they should be either a separate tag for unity's canvas and java's canvas or the description of the tag should include these 2 options too (which would mess up the "favourite tags" feature on the site though).