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"processed uncompressed bytes"/"all uncompressed bytes" is a distorted progress indication anyway.

Or put that length in the response headers.

You won't be able to do this if you're downloading from a CDN. Which is exactly where you would host large files, for which progress reporting really matters.

Why not? Every CDN I've ever worked with preserves custom headers.

Right. For example S3 supports custom headers, as long as that header happens to start with "x-amz-meta-..." - and now your progress reporting is tied to your CDN choice!

Not sure about you, but to me "XmlHttpRequest" in my request handling code feels less dirty than "x-amz-meta-". But to each their own I guess.

S3 is not a CDN.

Check CloudFlare, Akamai, ...

Good idea. We could give it a new MIME type too. E.g. application/octet-stream-with-length-prefix-due-to-idiotic-fetch-api

Or we can keep using XmlHttpRequest.

Tough choice.