Via Michael Pershan:
First, his construction with the Desmos API. I can’t wait to use this next year, and I’m working on my own using the Desmos API as an introduction to transformations before motion rules are introduced.
Second, this page from Khan Academy does much of what I’d like to do, but with a more specific intent — to get at 8.G.2 and 8.G.4, determining that two figures are congruent/similar based on whether one can be mapped to another using transformations. Does much of what I’d love my Desmos page to do, although I’m looking more for a user-friendly sandbox to explore transformations in a more open-ended way.