thanks for your reply and thanks a lot for Stack code.
I don't see exactly what you mean about embedding the value of a CAS variable in some HTML string (rather LateX code). I suppose you think it's possible to do some graphs in pure HTML code, but I don't know do that. In fact, doing some dynamics graphs with Stack and pstricks code is easy because Geogebra can export a file in pstricks. I only replace some caracters by CAS random variables in pstricks code generated by Geogebra. On the other hand, pstricks is well documented and it's possible to do such differents types of pictures. Doing some questions with dynamics graphs with Stack is a very cool feature.
P.S: I translate Stack in french with a friend. Not finished yet, but in progress.