Exemplars of current attempts to move towards the vision described above use technologies such as:

JavaScript, Java, VRML, plug-ins, Flash, Shockwave.

Commercial non-educational sites, such as Yahoo, Microsoft and a host of others, already make significant use of customizability. In general, sites on the cutting edge of web design technology have moved a long way from the original text and a few graphics of 15 years ago.
The third criterion, content, is still the most difficult to meet. Websites in general tend to be varied in quality and good exemplars are isolated. Though a particular site may include good individual examples of interactive mathematics, it is still rare to find extensive and numerous examples on one site.
Some current examples of good practice are reviewed here.

The first group describes the types of online technology available to the mathematics curriculum.
The second group gives examples of sites using this technology.