List numbering seems to cross-reference nested lists:

http://www.axeldahmen.de/Office/blog/en/msoft/ASP.NET-Core--Nothing-But-Chaos.html

As you can see from the screenshot, the heading (<h1>) doesn't continue the previous heading, but instead continues the nested ordered list.

Google Chrome MS Edge recognize the nesting correctly.

The Bugbug bot thinks this bug should belong to the 'Core::Layout: Generated Content, Lists, and Counters' component, and is moving the bug to that component. Please revert this change in case you think the bot is wrong.

Adding something like counter-reset: list-item to the root fixes it, which is a bit odd...

Regression window:
https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=5dc0652cd024fef30217ac3e6087b7a32f16967f&tochange=9bad25ccc8d0414e21d3c8d4dcf2178a97bb07c3

So, This is not a regression. The bug appear since Bug 1518201 landed.

Before Bug 1518201, "0." for all headings (<h1>).
After Bug 1518201, "A." and "C."

This is exactly the same as the first testcase, but with this addition:

::marker { content: counters(list-item, ".") }

That should obviously not make a difference to the numbering and it doesn't in Firefox, but it does in Chrome - they now agree with us on the numbering.

... and here's the same test using a foo counter instead of list-item. Again, it should have the same ordinals as the previous tests, and it does in both Firefox and Chrome.

We're implementing the spec correctly and Chrome is not. They apparently don't use the built-in list-item CSS counter for HTML lists (as required by spec), or they do but have added special non-compliant rules for it.

As noted by @emilio above, here's a workaround to avoid relying on Chrome bugs for this document:

:root { counter-reset: list-item; }

Marvellous!

Thank you @mats, for taking the time.