Because after you hover on something, it can be an indication of intent. Just loading an image when the webpage loads doesn't give you any insight into what the user 'interacted' with.
Yes, that's (one of the many reasons) why javascript needs to die.
> Just loading an image when the webpage loads doesn't give you any insight into what the user 'interacted' with.
That's what I said: regardless of whether the browser, when loading a page, does or does not include hidden images in what it fetches, that cannot possibly tell you anything about what the user interacts[0] with after the page has loaded and browser is no longer communicating with your server.
0: edit: obviously excluding clicking links or other navigation-causing interactions, but that's a separate problem.
On hover can be easily done with css.
selector:hover{background-image: url();}
That can trigger a tracking pixel.
> Yes, that's (one of the many reasons) why javascript needs to die.
You're missing the point. CSS allows you to load tracking pixels on hover, so you can do that without Javascript.
0: I decline to treat the unintentional flaws of [Firefox] as more important than the intentional ones.[1]