Courses/CS 461/Museum of unintended consequences/Counting web page visits
From CSWiki
It used to be more popular than it is now, but you still see lines on web pages saying something like: you are the XXXXth visitor since whenever. What's interesting about those lines is the means they use to count visitors.
The display of the count of the number of visitors is a graphic. Graphics are loaded separately from the rest of a page. Each graphic is loaded as an <img src = "URL" /> object. When a web page wants to count its visitors it signs up with a service and uses a special URL associated with that service as the src of the image. The service then generates a special graphic with a count of the number of times the graphic has been called for.
So the point is that the fact that images are loaded separately allowed companies to provide web visit counting services. That certainly wasn't the intended purpose of the <img ... > tag. But it's existence allowed a niche service to come into existence. The offerers of that niche service somehow figured out how to make money by offering that service — for free to the websites that used it.
