Add tests for 15.3.9 Margin Collapsing Quirks

[Psychpsyo]

This adds tests for this section of the html spec.
It currently passes in Firefox, but not in Chromium or Safari.

@bfgeek you might be interested, as I believe this is responsible for at least part of the developer confusion you point to in whatwg/html#10296

[bfgeek]

Yeah I don't really agree with these tests, or that section of the specification. It was written before we had the requirement of at least two implementations agree with the direction of the specification, and as you've discovered, implementations implement this quirk very differently.

IMO the path to compat is to reduce this quirk, not encode it.
If we want to encode it was need to specify either the Blink/WebKit method of how this works, or write out the UA-stylesheet rules that Gecko uses using standard CSS.

[Psychpsyo]

IMO the path to compat is to reduce this quirk, not encode it. If we want to encode it was need to specify either the Blink/WebKit method of how this works, or write out the UA-stylesheet rules that Gecko uses using standard CSS.

I do believe the Gecko styles for this are written out in standard CSS, here: https://searchfox.org/mozilla-central/source/layout/style/res/quirk.css

EDIT: just noticed it uses :-moz-last-node, sorry.

[bfgeek]

Yeah it doesn't use standard CSS - this is part of the problem. There are two different implementation methods each with different side effects. The easiest path to compat here is to slowly remove the quirk.

edit - it uses a lot of magic:

• -moz-only-whitespace
• -moz-first-node
• -moz-last-node