It uses SqueakJS so in theory it supports whatever SqueakJS supports: https://squeak.js.org/.

> SqueakJS executes Squeak in a web page without a plugin. It is a fully capable virtual machine implemented in pure JavaScript running unmodified Squeak images. Squeak is a modern implementation of Smalltalk, the original dynamic object-oriented programming environment. It runs bit-identically on virtually any platform, and now in the web browser, too.

I had not seen SqueakJS before, cool idea.

I have played with Amber, which is a mature Smalltalk development system in a browser experience.