MIDI hardware support has been available for a long time in Windows, OSX, iOS and most computer/tablet/smart phone platforms through USB, WiFi and even Bluetooth interfaces. But until now, there has been no standard mechanism to use MIDI devices with a Web browser or browser-based Operating System.
The Web Audio Working Group of the W3C has designed the Web MIDI API to provide support for MIDI devices as a standard feature in Web browsers and operating systems across multiple hardware platforms.
Google has led the way to support the inclusion of MIDI in the Web platform, both contributing to the specification and by shipping the first implementation of the Web MIDI API (in Chrome v.43 for Windows, OSX, and Linux), continuing to demonstrate the company's interest in helping musicians interact with music more easily using the Web.
Being able to connect to local MIDI hardware will increase the creation and distribution of music-making applications for PCs, tablets and smart phones. It also means that popular MIDI hardware can be used to control any kind of software in the browser (using physical buttons and knobs instead of on-screen sliders, for example).
For hardware device makers, instrument control panels and editor/librarians which previously needed to be produced in multiple versions can now be implemented once in HTML5, and consumers can run them on any Web device (tablet, computer, or smart phone) and even "live" over the Web.
And finally, since the browser is connected to the Internet, musicians can more easily share data and even connect music devices over a network.
Web MIDI has the potential to be one of the most disruptive music technology in a long time, maybe as disruptive as MIDI was originally back in 1983. Did Dave Smith and Ikutaro Kakehachi envision a world with 2,6 billion MIDI enabled smart phones. Definitely not!
Here is an interesting video of someone using a commercially available MIDI controller to play a browser based game. it just makes you think about all the possibilities in the future.
Here are some links to more Web MIDI resources.