Able to reproduce the issue on reported chrome version 64.0.3282.167 and latest canary 66.0.3350.0 using Mac 10.12.6, Windows-10 and Ubuntu 14.04 hence providing Bisect Info

Bisect Info:
================
Good build: 64.0.3280.0
Bad build: 64.0.3281.0

You are probably looking for a change made after 520004 (known good), but no later than 520005 (first known bad).

https://chromium.googlesource.com/chromium/src/+log/447db371759f0d9638769a7b22b9560a1b2d5da5..a4ebe08c91e29140e700c7bae9b94f27a786d1ca

Reviewed-on: https://chromium-review.googlesource.com/783911

Assigning it to nainar@chromium.org(reviewer), while assigning to meade@chromium.org(author) it is showing as "Not working on Chrome any more"

@Nainar: Please confirm the issue and help in re-assigning if it is not related to your change.
Adding ReleaseBlock-Stable as it is seems a receent break, feel free to remove it if not applicable.

Thanks!

M65 Stable promotion is coming VERY soon. Your bug is labelled as Stable ReleaseBlock, pls make sure to land the fix and request a merge into the release branch ASAP. Merge has to happen latest by 1:00 PM PT Tuesday (02/27/18) in order to make it to last M65 beta release next week. Thank you.

Gentle ping !!
Could someone from Blink>CSS team ,Please merge fix ASAP as M65 stable promotion is today night as it is marked stable blocker for M65.

Thanks..!

This is silly bug, and it works on chrome v63, but for v64 it's broken.
Please fix it ASAP.
Thanks.

Fix https://chromium-review.googlesource.com/c/chromium/src/+/935749 is in flight.

Thank you  dgozman@. Pls request a merge to M65 once cl listed at #9 is landed/baked/verified in canary and looks safe to merge. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8a4ccd6b71037a1452a2cb2466029e1cb17eed48

commit 8a4ccd6b71037a1452a2cb2466029e1cb17eed48
Author: Andrey Lushnikov <lushnikov@chromium.org>
Date: Mon Feb 26 16:52:41 2018

DevTools: fix rule insertion with file URLs.

This patch allows accessing rules for inspector style sheets.
Our tests didn't catch the regression since we run with
--allow-file-access-from-files flag.

Bug:  814937 
Change-Id: I58eabb8e299a81c5a912d5a42175ab78ad904704
Reviewed-on: https://chromium-review.googlesource.com/935749
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539166}
[modify] https://crrev.com/8a4ccd6b71037a1452a2cb2466029e1cb17eed48/third_party/WebKit/Source/core/css/CSSStyleSheet.cpp

Able to reproduce the issue on chrome reported version 64.0.3282.167
Verified the fix on Windows-10, Ubuntu 14.04, Mac 10.13.3 on Chrome version #66.0.3356.0 as per the comment#0
Attaching screen cast for reference.
Observed "Able to add new style rule to element"
Hence, the fix is working as expected.
Adding the verified label.

Thanks!

Deleted: 814937.mp4 2.5 MB

	814937.mp4 2.5 MB View Download

The NextAction date has arrived: 2018-02-27

The fix is safe, I didn't find any recent crashes with CSSStyleSheet::CanAccessRules in stack trace, so requesting merge to M65.

This bug requires manual review: We are only 6 days from stable.
Please contact the milestone owner if you have questions.
Owners: cmasso@(Android), cmasso@(iOS), bhthompson@(ChromeOS), govind@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Approving merge to M65 branch 3325 based on comments #13 and #15. Please merge ASAP. Thank you.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/31b082a8b62da9d11f02dedc477911189c8ff533

commit 31b082a8b62da9d11f02dedc477911189c8ff533
Author: Dmitry Gozman <dgozman@chromium.org>
Date: Tue Feb 27 18:11:21 2018

DevTools: fix rule insertion with file URLs.

This patch allows accessing rules for inspector style sheets.
Our tests didn't catch the regression since we run with
--allow-file-access-from-files flag.

TBR=lushnikov@chromium.org

(cherry picked from commit 8a4ccd6b71037a1452a2cb2466029e1cb17eed48)

Bug:  814937 
Change-Id: I58eabb8e299a81c5a912d5a42175ab78ad904704
Reviewed-on: https://chromium-review.googlesource.com/935749
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#539166}
Reviewed-on: https://chromium-review.googlesource.com/939763
Cr-Commit-Position: refs/branch-heads/3325@{#607}
Cr-Branched-From: bc084a8b5afa3744a74927344e304c02ae54189f-refs/heads/master@{#530369}
[modify] https://crrev.com/31b082a8b62da9d11f02dedc477911189c8ff533/third_party/WebKit/Source/core/css/CSSStyleSheet.cpp

Able to reproduce the issue on win-10 using chrome reported version #64.0.3282.167.

Verified the fix on Mac 10.13.3, Win-10 and Ubuntu 14.04 using Chrome version #65.0.3325.106 as per the comment #0.
Attaching screen cast for reference.
Observed that devTools add "add new style rule" for local webpages.
Hence, the fix is working as expected. 
Adding the verified labels.

Thanks...!!

Deleted: 814937.mp4 877 KB

	814937.mp4 877 KB View Download

 Issue 817959  has been merged into this issue.

 Issue 818518  has been merged into this issue.

 Issue 814570  has been merged into this issue.