This is a followup to
#23808

css/css-backgrounds/border-image-slice-005.htm
css/css-backgrounds/border-image-slice-007.htm
css/css-backgrounds/reference/border-image-slice-005-ref.html
css/css-backgrounds/reference/border-image-slice-007-ref.html
css/css-backgrounds/support/9grid40-30-20-10-green.png
css/css-backgrounds/support/9grid40-30-20-10-red.png

Over in PR23808, these were @fantasai's most relevant+important review comments:

{
(...) We need an image that doesn't only detect when the UA slices too much, but also if it slices off too little.

My suggestion here is to use a skewed "striped border": design the image so that once it is sliced and squished into the border, it creates an evenly striped border. Since the numbers here are all divisible by a factor of 5, then 5 even stripes on each side would work well. The image will be uneven from side to side, but once it is squished into the border it will be even all the way around.

If there are stripes missing in the test, then we know the UA sliced too little of the image. If there is red in the test, then it sliced too much. It has to slice exactly the right amount.
}

On my website, the tests and references are:

http://www.gtalbot.org/BrowserBugsSection/CSS3Backgrounds/border-image-slice-005-newest2-GT.html

http://www.gtalbot.org/BrowserBugsSection/CSS3Backgrounds/reference/border-image-slice-005-newer-GT-ref.html

http://www.gtalbot.org/BrowserBugsSection/CSS3Backgrounds/border-image-slice-007-newest2-GT.html

http://www.gtalbot.org/BrowserBugsSection/CSS3Backgrounds/reference/border-image-slice-007-newer-GT-ref.html

I must add that it seems to me impossible to avoid some fuzzy anti-aliased pixels around the leftmost vertical blue stripes when zooming/magnifying to, say, 200% the webpages of tests. Without zooming/magnifying, I believe that a normal human can not, will not see fuzziness around the leftmost vertical blue stripes.