While browsing this site, I was surprised to find that TeX allows one to produce various kinds of output that could not be represented on paper. For example, one has animation ( How can we draw Christmas animations with TikZ?) and icons in PDFs that generate pop-ups when clicked on (the
pdfcomment package). What other features of this kind are there?
(The specific thing that led me to ask this q. was a desire to be able to expand/compress sections of output text, in order to be able to view a document in different 'levels of detail'. I suspect that this is not possible because it would interfere with page breaking algorithms -- but then, I wouldn't have thought animations were possible in TeX either!)
Edit: knowing about PDF viewers' support levels for such features would also be welcome. I should also say that I'm using enough of TeX's typesetting features that e.g. webpages come nowhere near being a viable alternative.