Do we use word counts? Paragraph counts? Visual scroll position? Do we rely on markup/DOM characteristics of Readability? If so, what do we do in the future if that changes? Are we walking DOM text nodes, DOM paragraph nodes, or something else? What can we do that's fairly stable across desktop, Android, and iOS? This is a firm dependency for Bug 857990, and it also affects what we choose to do for word counts (in conjunction with a decision on reading time estimation -- Bug 959297, Bug 1106380). If we decide that this is hard, or we don't care about either scroll position or word count/reading time yet, we can punt this. But we should be aware that this complicates future extension, and is a parity feature.