UserAgent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:50.0) Gecko/20100101 Firefox/50.0

Example URL:
http://www.gtalbot.org/BrowserBugsSection/CSS3WritingModes/s74-block-normal-flow-overconstrained-vrl-004.xht

Steps to reproduce the problem:
Load these 2 reduced self-explanatory tests:

(vertical-rl)
http://www.gtalbot.org/BrowserBugsSection/CSS3WritingModes/s74-block-normal-flow-overconstrained-vrl-004.xht

and

(vertical-lr)
http://www.gtalbot.org/BrowserBugsSection/CSS3WritingModes/s74-block-normal-flow-overconstrained-vlr-005.xht

What is the expected behavior?
A filled green square and no red

What went wrong?
A filled green square and a filled red square

Does it occur on multiple sites: N/A

Is it a problem with a plugin? No 

Did this work before? No 

Does this work in other browsers? Yes

Chrome version: 57.0.2986.0  Channel: dev
OS Version: 
Flash Version: Shockwave Flash 11.2 r202

The 2 tests have detailed and carefully edited comments. 

Basically, these 2 tests are specifically testing the following chunk of the CSS2.1 spec when applied to a vertical writing-mode context:

{
'margin-left' + 'border-left-width' + 'padding-left' + 'width' + 'padding-right' + 'border-right-width' + 'margin-right' = width of containing block

 If all of the above have a computed value other than 'auto', the values are said to be "over-constrained" and one of the used values will have to be different from its computed value. If the 'direction' property of the containing block has the value 'ltr', the specified value of 'margin-right' is ignored and the value is calculated so as to make the equality true. If the value of 'direction' is 'rtl', this happens to 'margin-left' instead.
}
CSS 2.1, section 10.3.3 Block-level, non-replaced elements in normal flow
http://www.w3.org/TR/CSS21/visudet.html#blockwidth

I will submit those 2 tests to the CSS Writing-Modes test suite eventually, along with the -002 and -003 tests (testing 'direction: ltr').