I went to a reactjs meetup at Apple a few days ago. There they showed off Revis, a react library for making charts and visualizations that looked really interesting, with the selling point being that it enables you to create graphs declaratively. They said they had plans of open sourcing it, although they didn't give a due date.

Another thing they indirectly showed off was a reactjs playground which was very similar to swift playgrounds. I say indirectly, because the person that was presenting was showing revis using it. There's some public playground components that are around (e.g. component-playground [0]), but it's the type of thing where I think there's still room to experiment and innovate. Bret Victor's Inventing on Principle video [1] comes to mind.

I don't know how big the web presence at Apple has been in the last few years, but I'm hopeful that this is a sign that it's growing and that it'll push more resources to the Safari team so it'll keep up better with other browser vendors. Although they have a history of open source, I don't think they've open sourced much JavaScript? From googling around all I could find was that they were one of the biggest contributors to SproutCore [2], which looks fairly dated by today's standards.

[0] http://projects.formidablelabs.com/component-playground/

[1] https://vimeo.com/36579366

[2] http://sproutcore.com/