When you encounter one of these, first try turning off CSS styles (View -> Page Style -> No Style). If that fixes the issue then the page author did something

When you encounter one of these, first try turning off CSS styles (View -> Page Style -> No Style). If that fixes the issue then the page author did something in their styles to mess with scrolling. If that does not work, try turning off javascript (NoScript extension works for this). If that fixes it, then the page author is doing something in their javascript code that is breaking keyboard scrolling. As for "what to do" - other than complain to the page authors, not much beyond turning off CSS and/or Javascript to get things to work better.