UserAgent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Firefox/52.0

Example URL:
http://www.gtalbot.org/BrowserBugsSection/CSS3Backgrounds/box-shadow-032.xht

Steps to reproduce the problem:
2 reduced self-explanatory tests that Chrome 65.0.3315.3 (beta release) fails:

http://www.gtalbot.org/BrowserBugsSection/CSS3Backgrounds/box-shadow-032.xht
which uses the code 
box-shadow: green inset 100px 100px 0px 0px;

http://www.gtalbot.org/BrowserBugsSection/CSS3Backgrounds/box-shadow-036.xht
which uses the code
box-shadow: 100px 100px 0px 0px inset green;

Firefox 60.0a1 buildID=20180210220139 passes those 2 tests.

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

What went wrong?
A 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: <Copy from: 'about:version'>  Channel: beta
OS Version: 
Flash Version: irrelevant

The 'box-shadow' syntax is

<shadow> = inset? && <length>{2,4} && <color>?

https://www.w3.org/TR/css-backgrounds-3/#the-box-shadow

therefore the optional reserved keyword inset and the optional color could be anywhere, in any order, except between length values.

The buggy behavior (red square) remains if I use 2 (or 3) length values instead of 4. Eg:
box-shadow: green inset 100px 100px;
fails in Chrome 65.0.3315.3.

Tests pass if color is defined with the color property, outside the box-shadow declaration.
Eg
color: green;
box-shadow: inset 100px 100px;
creates a green shadow overlapping the red square in Chrome 65.

- - - - - - - 

Chrome 65.0.3315.3 also fails

http://test.csswg.org/suites/css-backgrounds-3_dev/nightly-unstable/html4/box-shadow-syntax-001.htm

for the exact same (parsing) reasons.