That doesn't always work, since the page can intercept the key presses and do whatever it wants with them, and also because the input focus may be inside some element on the page, such as an embedded video, requiring a click somewhere else on the page to return the focus to the entire document.

You know those little arrow buttons at each end of the scroll bar? The ones that scroll the content one line at a time? Gone as of Chrome 32. Anyone else think this is a terrible idea? Bug report here.