These are all things we have something to say about. For instance, the types of visual impairment vary so much that no general advice on typography for this group will be universally useful. Situations of use vary, too, and are usually impossible or very difficult to predict. At the 'micro' level, the ways in which the features of a document interact make it hard to give advice about a particular feature. Despite a century of research into legibility, readability, comprehension and effective learning strategies, we don't know enough about what makes good documents.
Good documents are still partly an act of magic.
They are made by good writers and good designers rather than simply by reference to research results. All this means that the briefings we offer on this site are limited and in some situations will be wrong. Speak up, and try not to bump into the furniture', he is said to have offered. Our briefings offer advice at the same level. We hope you find them useful.