This is Maxima [1], a computer algebra system.

The browser frontend has been built by Marius Gerbershagen using Embeddable Common Lisp [2], emscripten [3], gnuplot [4] and MathJax [5].

Source code available on https://gitlab.com/spaghettisalat/maxima/-/tree/emscripten-port-deployed?ref_type=heads
[1] https://sourceforge.net/projects/maxima/
[2] https://ecl.common-lisp.dev/
[3] https://emscripten.org/
[4] http://www.gnuplot.info/
[5] https://www.mathjax.org/

Announced by Zach Beane on https://lispblog.xach.com/post/773844754188419072