Mathematics on paper looks nice, but it is not interactive.

Dead Math on webpages - math symbols and graphs made of GIFs or JPGs - is not interactive.

If you have a 3D graph on a webpage, you should be able to pick it up and spin it, looking inside, from a different angle, on top, from the bottom.

If you have an algebraic or numeric computation on a webpage, you should be able to change the inputs, and experiment with the computation yourself.