Web MIDI API
Lets a web page talk directly to MIDI devices plugged into the computer — keyboards, synths, control surfaces.
Calling navigator.requestMIDIAccess() returns lists of inputs and outputs, and from there it is raw MIDI bytes in both directions. Chrome shipped it in 2015, Firefox only in 2022 and even then gated behind a site-specific add-on the page has to request. Safari has declined, citing fingerprinting: the list of attached devices says a fair bit about who you are.
SysEx needs an extra permission, {sysex: true}, for good reason — SysEx can rewrite an instrument's memory or flash its firmware. Still, the API has turned the browser into a perfectly capable patch editor for old synths whose Windows 98 software stopped working long ago. It is often paired with the Web Audio API to build synths in a tab.