Cristiano Belloni has relaunched KievII Host as Hya.io. This is a web-based plugin-based audio application that supports hardware MIDI though Web MIDI, audio synthesis, sequencers, and lots of plugins.
The project has a blog with announcements about updates, plugins, and some technical background. The web interface is closed source, but the plugins Cristiano has written are open source. For example, hy-osc is an oscillator, and kmx is a mixer.
Hya supports AMD and ES6 modules. They use a simple JSON object that describes the plugin, and a constructor function. You can bind to the Hya GUI using various events, like handleMouseOver and handleMouseDown. The post Developing a canvas plugin for Hya.io explains how to create plugins in detail.
The documentation has details on MIDI interaction and the initPlugin constructor. It's amazing to think that you could connect a MIDI control surface or keyboard to a web application.