It's not enforced for quirks mode per the HTML Standard. I suspect it would break too much to remove that quirk. Might be worth investigating again, but maybe separately from fixing the nosniff issue?

Ah. The quirk is only for same-origin stylesheets. I see. I guess that's quite a bit less risky than I was thinking. :)

I'll add some cross-origin checks to that test, and see if I can slap nosniff support together. Shouldn't be too hard.

I jumped on this yesterday because it looked trivial. That'll teach me... Ah well. Intent to ship at https://groups.google.com/a/chromium.org/d/msg/blink-dev/A_ICVgLl2kQ/mrD6bb2wBAAJ.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/722937f7116a2c448d0700b6d90318852369c785

commit 722937f7116a2c448d0700b6d90318852369c785
Author: Mike West <mkwst@chromium.org>
Date: Mon Nov 27 11:59:08 2017

Implement 'nosniff' support for stylesheets.

Spec: https://fetch.spec.whatwg.org/#should-response-to-request-be-blocked-due-to-nosniff?
Intent to Ship: https://groups.google.com/a/chromium.org/d/msg/blink-dev/A_ICVgLl2kQ/mrD6bb2wBAAJ

Bug:  788096 
Change-Id: I05e30696f5d072892a3de472654dbf464483313b
Reviewed-on: https://chromium-review.googlesource.com/788384
Commit-Queue: Mike West <mkwst@chromium.org>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519281}
[delete] https://crrev.com/5e3ea73074e0d9c7455b3d8e40b938a7ace7231d/third_party/WebKit/LayoutTests/external/wpt/fetch/nosniff/stylesheet-expected.txt
[modify] https://crrev.com/722937f7116a2c448d0700b6d90318852369c785/third_party/WebKit/Source/core/loader/BaseFetchContext.cpp
[modify] https://crrev.com/722937f7116a2c448d0700b6d90318852369c785/third_party/WebKit/Source/core/loader/BaseFetchContext.h
[modify] https://crrev.com/722937f7116a2c448d0700b6d90318852369c785/third_party/WebKit/Source/platform/loader/fetch/FetchContext.h
[modify] https://crrev.com/722937f7116a2c448d0700b6d90318852369c785/third_party/WebKit/Source/platform/loader/fetch/ResourceLoader.cpp
[modify] https://crrev.com/722937f7116a2c448d0700b6d90318852369c785/third_party/WebKit/Source/platform/loader/fetch/ResourceRequest.h
[modify] https://crrev.com/722937f7116a2c448d0700b6d90318852369c785/third_party/WebKit/Source/platform/loader/testing/MockFetchContext.h