The page says "White at a next is considered the main continuation"--but the continuations after a aren't discussed! There's one variation at 4-4 point staircase joseki, but I'm sure there are simpler lines that are worth mentioning (as well as more complicated things that I'm not ready to study just now...) Maybe they are on a different page that I haven't managed to find yet?

You're right, I should have tried that :-)

However, it's rather confusing, since the other variations can't be reached by clicking on the diagram, but only by following the text links next to the second diagram.