curl -H "Content-Type: text/html; charset=utf-8" \
    --data-binary @FILE.html \
    https://validator.w3.org/nu/?out=gnu

lord, what is all this stuff?

   Compiling proc-macro2 v1.0.60
   Compiling quote v1.0.28
   Compiling libc v0.2.146
   Compiling parking_lot_core v0.9.8
   Compiling lock_api v0.4.10
   Compiling serde v1.0.164
   Compiling syn v1.0.109
   Compiling memchr v2.5.0
   Compiling tokio v1.28.2

What part of the build needs to be compiling binaries?

Due to whatever that stuff is, my local spec build is failing:

  = note: ld: library 'System' not found
          cc: error: linker command failed with exit code 1 (use -v to see invocation)

So, trying a remote spec build now

That is what seems to follow from https://fetch.spec.whatwg.org/#ref-for-concept-http-network-or-cache-fetch, where this part comes first:

Set response and internalResponse to the result of running HTTP-network-or-cache fetch given fetchParams.

…which is what calls into https://fetch.spec.whatwg.org/#ref-for-response.cacheability, which says this:

Store httpRequest and forwardResponse in httpCache, as per the "Storing Responses in Caches" chapter of HTTP Caching. [HTTP-CACHING]

But, all that happens before https://fetch.spec.whatwg.org/#ref-for-redirect-status②, which is this:

If internalResponse’s status is a redirect status: … Set response to the result of running HTTP-redirect fetch given fetchParams and response.