WPT details: wpt/fetch/ wpt/service-workers/
Fetch upload streaming lets web developers make a fetch with a ReadableStream body. Fetch provides a generic definition of Request and Response objects (and other things involved with network requests).
Traditionally uploading byte arrays were implemented on WebSocket but it has made client/server code messy. Fetch upload streaming makes that simpler.
Docs: http://bit.ly/3asqra2
Explainers: https://bit.ly/2SVvKbR