I'm so glad you spotted that mostly everyone else just pretended they knew what that was. It is the most obscure CSS you can imagine. Consider your desktop walpaper, now remove the image, you will notice a colour. Black blue red yellow. background: background; uses the OS background colour as a CSS property. The important is.. just for fun.
You know, I did know that, but it was burred deep in my brain. Now that you explain it, I remember you explaining it to me in person last year! It is a really unusual thing...I'm guessing that it is a throwback to a really creaky old behaviour, as opposed to a new exciting feature!
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