A web page is built for a person with a mouse, not a display on a wall. It asks you to log in, shows a cookie banner, or puts the part you care about behind a click. Left alone, a screen would show all of that, clutter and all.
Running your own code on the page fixes it. As a page loads, Screenly can run a script you provide, a technique called JavaScript injection, so the screen can sign in, dismiss the banner, hide the navigation, or scroll to the panel that matters. A page made for people ends up behaving the way a screen needs.
This is what turns “almost the right thing” into exactly what you want on the wall. The page is not yours to change at its source, but what the screen does with it is.