#3223 is part of this, but to properly integrate with Fetch we need more. In particular, I think we want a serialization operation that returns a tuple. The tuple contains the boundary and a list of which each item is either a byte sequence or a Blob. That allows Fetch to compute the total size (go through the list, and increment by either byte sequence's length or blob's size) and allows it to enqueue chunks into a stream lazily without blocking I/O. It's not really possible to pretend synchronous I/O and allow user agents optimize as the I/O might fail, whereas obtaining the size should not fail (thanks to @mkruisselbrink for pointing that out).

We should also point out that this is a potentially lossy format as the boundary needs to be necessarily computed ahead-of-time without knowing the contents of the blobs. There is no way to avoid this as the boundary is part of the headers and exposed through something like new Response(formData).headers.get("content-type"). I suppose it was possible to avoid this before there was an API if you did not care about streaming, but here we are.

There's a separate question of where we want to define this format. At the moment it's mostly in HTML but FormData is in XMLHttpRequest. Status quo is fine with me.

cc @andreubotella