Based on the description of the canvas tag ...
A new feature of HTML5, allowing Javascript apps to have a drawing surface in the browser.
...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).