Why would you want MIDI in your Web browser?
This will allow developers to create new types of applications – like browser-based synthesizers – that you can control with a MIDI keyboard. Or a developer could create a Web based music sequencer. You could even create a MIDI controller UI that works through your Web browser.
The Web MIDI API
The Web MIDI API is intended to enable direct access to devices that respond to MIDI – external synthesizers or lighting systems, for example, or software synthesizers. The Web MIDI API is also explicitly designed to enable a new class of applications on the web that can respond to MIDI controller inputs – using external hardware controllers with physical buttons, knobs and sliders (as well as musical controllers like keyboard, guitar or wind instrument controllers) to control web applications.
The specification defines an API supporting the MIDI protocol, enabling web applications to enumerate and select MIDI input and output devices on the client system and send and receive MIDI messages. It is intended to enable non-music MIDI applications as well as music ones, by providing low-level access to the MIDI devices available on the users’ systems.
Details are available at the W3 site.