The Serial API will provide a way for documents to interface with serial ports through script.
The idea behind it is to bridge the web and the physical world, and communicate with devices such as Arduinos, 3D printers, and other serial devices. This is currently only available to desktop apps, or to websites through the use of insecure and outdated technologies (Java Applets, NPAPI plugins).
The Serial API could be implemented using the serial library by wjwwood, also used in ROS. It's a very good, open-source, stable C++ library with a clear documentation, and it's MIT-licensed.