Thanks for the report, sounds like a reasonable change to me.  loonybear@, could you please take a look?

Perhaps we could add a the milestone / goo.gl link to docs at the end of the message:
https://sites.google.com/a/chromium.org/dev/Home/chromium-security/deprecating-permissions-in-cross-origin-iframes
https://github.com/WICG/feature-policy/issues/78#issuecomment-316097292

Sounds even better to me !

Thanks for reaching out! 
Yes I totally agree that the message should be more descriptive. I am hesitant on the milestone link cause the 1st link is for permission only and the 2nd link looks less formal.
Ian, what do you think about adding a link to the message?

Agree that we should change something :)

The last responses to the Intent to Deprecate thread (https://groups.google.com/a/chromium.org/d/msg/blink-dev/QL_3En5aq70/k1jz50n5CQAJ) suggested removing immediately. However, if this is actually getting use, we should add a milestone, "Soon" is pretty vague.

What do you think about adopting something like the original suggestion, with a real milestone:

"The old syntax (allow="feature1 feature2 feature3 ...") is deprecated and will be removed in Chrome 68. Use semicolons to separate features (allow="feature1; feature2; feature3")"

I just checked on chromestatus.com, the usage for the deprecated syntax is 0.000458%

https://www.chromestatus.com/metrics/feature/timeline/popularity/2145

I will update the message for now and remove it for 68. 

Thanks

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0d5b36f3f235972defc3c560db69c8529e2d11d5

commit 0d5b36f3f235972defc3c560db69c8529e2d11d5
Author: Luna Lu <loonybear@chromium.org>
Date: Wed Feb 14 22:29:23 2018

Update error message for feature policy deprecated syntax

Bug:  785816 
Change-Id: Id4eda7ecfa7b184f81d1b827e79d8cf3e14a9845
Reviewed-on: https://chromium-review.googlesource.com/919491
Commit-Queue: Luna Lu <loonybear@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536851}
[modify] https://crrev.com/0d5b36f3f235972defc3c560db69c8529e2d11d5/third_party/WebKit/Source/platform/feature_policy/FeaturePolicy.cpp

Tested this issue on Windows 10 on the reported version 62.0.3202.94 and latest Canary 66.0.3348.0 by following the steps mentioned in the original comment.
Able to reproduce this issue on the reported version 62.0.3202.94 and the issue is fixed on the latest Canary 66.0.3348.0.

On loading the html page with iframe having attributes allow="microphone camera", can see the new error message on console as expected.
Attached is the screen shot and the test file for reference.

Hence adding TE verified labels as the fix is working as intended.

Thanks..

Deleted: test.html 69 bytes

test.html 69 bytes View Download

Deleted: 785816-M66-CL.PNG 38.7 KB

	785816-M66-CL.PNG 38.7 KB View Download

Close verified. Thanks susan.boorgula@!