Using the spacebar to scroll down and tab-spacebar is pretty much the only portable way to scroll in a browser. Make sure that the focus is not on an entry like a check box, text box, or button, and then push space - tada! The page scrolls until the last couple lines at the bottom of the screen are at the top.
If you prefer to scroll a few lines at a time, instead of in big chunks like this, you'll want to add key handlers for spacebar and tab-spacebar. Something like:
window.onkeydown = function(e){
if (e.keyCode == 32){ // If spacebar keycode
if (event.shiftKey) { // If shift is being held
window.scrollBy(0, -50); // Scroll over 0 pixels and up 50 pixels
} else { // Otherwise
window.scrollBy(0, 50); // Scroll over 0 pixels and down 50 pixels
}
}
};
would do this if you stick it into an extension. If you want this prepackaged, there are a few keyboard shortcut managers such as this one that would do the job.