Basically, Streaming Fetch
Historically, most progress bars for content loading type activity have been illusions: animations which fill up over a predetermined duration by the designer that do not actually relate to data being transferred at all. I've built some of these myself.
With ReadableStreams in the WHATWG's Fetch API, we're able to have real deal progress bars.
Check it out! Enter an Image URL below (preferably of a LARGE uncached image or something), and watch the progress bar ACTUALLY REFLECT the load status! This is so cool!
Sample image URLs are provided to the right. Simply click in the input field to copy the URL and paste it in the URL input field.
Further Reading: WhatWG Spec