Able to reproduce the issue on Windows 10, Mac 10.12.6, Ubuntu 14.04 on stable 61.0.3163.100. Issue is a non regression as observed from M50 builds 50.0.2624.0. Issue is not observed on FF browser.

Untriaged it so that issue gets addressed.

Ian, you've spent more time thinking about margins lately than anyone really should. What's your take on this?

FF is correct here.

We actually have a TODO[1] in LayoutNG to fix this (it will be a trivial change), but we haven't done this yet as we noticed that Edge didn't support it, and didn't know how web compatible this would be.

[1] https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc?q=ng_block_layout_al&sq=package:chromium&l=394 

A few different options:
 - Try a fix in existing layout margin collapsing code. See if we can land it without web compat issues. I had a quick look (~15mins), and its not immediately obvious how to fix this in the existing layout code).
 - Fix in LayoutNG now, and see if we can do this in LayoutNG phase 1 landing. 
 - Try to fix this after landing LayoutNG phase 1. 

Thoughts?

(also, thank you peter for the small testcase). :)

Option two would be ideal if it's web-compat. Option three if not.
Thanks!