The web version of the Molecular Materials Informatics toolkit has been in progress for many years, and went open source not that long ago, after being ported to TypeScript. The change of language basis makes the code look a lot more like the Java & Swift versions, and is a great deal more maintainable than any of the other options for creating sophisticated client-side apps for the web runtime.
The toolkit has its own interactive sketcher, which can be embedded into any web page, as long as the GPL 3.0 rules are abided by. There is now a nicer way to see it in action:
The page itself serves as an example of how to embed the sketcher within a web page and make use of basic functionality, i.e. grabbing the sketched molecule and doing something with it.