Tested the issue on 66.0.3359.117 using Windows 10, Mac 10.13.3 with steps mentioned below. Pasted fetch('.', {headers: {'X-Test': '123'}, keepalive: true}) in devtools console and observed error as VM87:1 Preflight request for request with keepalive specified is currently not supported
(anonymous) @ VM87:1
Promise {<rejected>: TypeError: Failed to fetch
    at <anonymous>:1:1}
VM87:1 Uncaught (in promise) TypeError: Failed to fetch
    at <anonymous>:1:1

@Reporter: Could you please check the screenshot and let us know if this is the behavior you are seeing? If not please provide a sample URL or test file to test this issue. 

Thanks!

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	835821.png 241 KB View Download

Yes, that is the behaviour being observed. I've also attached a test file that demonstrates the issue.

Deleted: fetch_keepalive_same-origin_no-cors.html 190 bytes

fetch_keepalive_same-origin_no-cors.html 190 bytes View Download

Thank you for providing more feedback. Adding the requester to the cc list.

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

Checked the issue on reported chrome version 66.0.3359.117 using Windows 10 with the below mentioned steps.
1. Launched chrome
2. Downloaded "fetch_keepalive_same-origin_no-cors.html" file provided in comment#3 and opened it in a new tab.
3. Opened DevTools
It shows an error "Fetch API cannot load". Attaching the screen shot of the same.

@Reporter: Could you please have a look at the screen shot and let us know if we have missed anything in the process. It would be highly helpful if confirmed on, whether the attached screen shot has an issue.

Thanks!

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	835821.PNG 257 KB View Download

The sample needs to be hosted on a webserver, it cannot be served by the filesystem. The error message shown in the screenshot of comment #2 is the error message in question.

Thank you for providing more feedback. Adding the requester to the cc list.

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

Tested the issue on chrome reported version 66.0.3359.117 using Windows 10 with steps mentioned below:
1) Launched chrome reported version, created a local server
2) Dragged and dropped the file provided in comment#3 and opened Devtools, seen the same error as mentioned in comment#5 screenshot

@Reporter: Please find the attached screencast for your reference and let us know if we missed anything in reproducing the issue, provide your feedback on it which help in further triaging it.

Thanks!

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	835821.mp4 2.7 MB View Download

fetch('.', {headers: {'X-Test': '123'}, keepalive: true}) can cause preflight due to redirect. It may be reasonable to relax the restriction for requests with mode: 'same-origin'.

You are still running the script from a file:// schema instead of an http:// or https:// as the error message in your video demonstrates. You need to host the example file on an actual web server such that you navigate to it via something like:

http://localhost/fetch_keepalive_same-origin_no-cors.html

If the URL bar in the browser shows file:// then you won't be able to reproduce the issue in this bug.

To move things along I've created a jsfiddle to demonstrate the bug in this issue: 

https://jsfiddle.net/pv8up9gd/