UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2899.0 Safari/537.36

Steps to reproduce the problem:
1. Install developer extension in reduced test case. 
2. Load a page, such as https://dosaygo.com or https://abc.xyz/
3. Observe the following ( summarized ) incorrect document tree:

html
- head
- body
- head
- body

4. Comment out the manifest.content_scripts[ 0 ].run_at directive in manifest.json so the content script is run at default of document_idle, instead of document_start, and Observe the normal expected document tree.

What is the expected behavior?
Document tree does not contain duplicate head and body elements if content script operates at document start.

Normal behavior observed when content script is run at default of document idle.

What went wrong?
Duplicate head and body elements ( actually they are not exact duplicates, but there are 2 head and 2 body elements ) when content script is run at document_start.

Maybe the HTML import load, template injection, and content script run, and actual owner document DOM content load, all interleave and so the documentElement is incorrectly given two head and body elements.

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 56.0.2899.0  Channel: n/a
OS Version: 10.0
Flash Version: Shockwave Flash 23.0 r0
 

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