Thanks for filing the issue!

Unable to reproduce the issue on reported chrome version 65.0.3325.146 using Mac 10.13.1 with the below mentioned steps.
1. Opened test.html in a new tab
2. Focused on content editable area
3. Pressed backspace, left and right arrow keys
We didn't observe any of those keys are working. Attaching the screen cast of the same.

@ilchenearly: Could you please have a look at the screen cast and let us know if we have missed anything in the process. Any further inputs from your end may help us.

Deleted: 821837.mp4 6.3 MB

	821837.mp4 6.3 MB View Download

I attached a video.

If I open the local test.html directly in Chrome, key events are prevented correctly. (like your screen cast)

But if I serve the test.html on local server (I use httpster for this), the key events are no longer prevented.

As you can see from the video, the two tab contains SAME html code, but result is different.

Deleted: Screenflick Movie 2.mp4 3.2 MB

	Screenflick Movie 2.mp4 3.2 MB 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

Sorry, new update:

Even I serve the html in my local server, the key events can be prevented correctly if I open it in incognito mode.

After trying each extensions I install, I found it is caused by Tampermonkey

So please close this, this is not a Chrome bug. Sorry for this report.

Thanks reporter for confirming the extension issue. Closing as per request.