Using the Web developer tools built into any web browser, you can edit the HTML of the page you are currently looking at, but that only affects what happens on your own computer, and it will all get reset if you just reload the page.
Actually, it is a very useful trick sometimes. I often do it when I am taking screen-grabs to go in a presentation, or if I want to mock-up some user-interface I am planning to implement.
But that could explain this screen-grab.