I have a patch in CQ to fix this for the top-level stylesheets. Nested imports from stylesheets still use the request URL. But the code can't be tested until service workers intercept sync requests, since these are sync. WIP patch for nested imports is at https://chromium-review.googlesource.com/c/chromium/src/+/1375180

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/defd74f29c29ff8563c0964ae7ff922372a8e31d

commit defd74f29c29ff8563c0964ae7ff922372a8e31d
Author: Matt Falkenhagen <falken@chromium.org>
Date: Thu Dec 13 08:38:15 2018

service worker: XSLT: Use response URL for the base URL.

This aligns with the standard. See
https://github.com/whatwg/fetch/pull/146

Chrome Status: https://www.chromestatus.com/feature/4857368017895424

No intent to ship because it's a trivial change.

Bug:  914135 
Change-Id: I229aec6f8473bb6b7cdc88429afa830bc6eb80ed
Reviewed-on: https://chromium-review.googlesource.com/c/1372109
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616248}
[modify] https://crrev.com/defd74f29c29ff8563c0964ae7ff922372a8e31d/third_party/blink/renderer/core/dom/processing_instruction.cc
[add] https://crrev.com/defd74f29c29ff8563c0964ae7ff922372a8e31d/third_party/blink/web_tests/external/wpt/service-workers/service-worker/resources/import-relative.xsl
[add] https://crrev.com/defd74f29c29ff8563c0964ae7ff922372a8e31d/third_party/blink/web_tests/external/wpt/service-workers/service-worker/resources/xsl-base-url-iframe.xml
[add] https://crrev.com/defd74f29c29ff8563c0964ae7ff922372a8e31d/third_party/blink/web_tests/external/wpt/service-workers/service-worker/resources/xsl-base-url-worker.js
[add] https://crrev.com/defd74f29c29ff8563c0964ae7ff922372a8e31d/third_party/blink/web_tests/external/wpt/service-workers/service-worker/resources/xslt-pass.xsl
[add] https://crrev.com/defd74f29c29ff8563c0964ae7ff922372a8e31d/third_party/blink/web_tests/external/wpt/service-workers/service-worker/xsl-base-url.https.html

Done for top-level imports, nested imports are tracked at issue 915124 (so far it doesn't matter because nested imports skip the service worker).