[cssom] Shortest serialization principle with multiple minimal possibilities

https://drafts.csswg.org/cssom/#serialize-a-css-value

If component values can be omitted or replaced with a shorter representation without changing the meaning of the value, omit/replace them.

But there are 2 different minimal ways to represent list-style: outside none disc:

• list-style: outside
• list-style: disc

And there are 3 different minimal ways to represent border: medium none currentcolor:

• border: medium
• border: none
• border: currentcolor

And there are 4 different minimal ways to represent text-decoration: none solid currentcolor auto:

• text-decoration: none
• text-decoration: solid
• text-decoration: currentcolor
• text-decoration: auto

And so on with lots of shorthands. Which minimal serialization should be picked?

var {style} = document.createElement("div");
style.listStyle = "outside none disc";
style.listStyle; // Firefox: "outside". Blink/WebKit: "outside none disc"
style.border = "medium none currentcolor";
style.border; // Firefox: "medium none". Blink/WebKit: "medium none currentcolor"
style.textDecoration = "none solid currentcolor auto";
style.textDecoration; // Firefox: "none". Blink: "none solid currentcolor". WebKit: "" (CSS3 syntax not implemented)

Start with the CSSOM “serialize a CSS value” section linked in the issue and compare its shortest-serialization rule with the listed list-style, border, and text-decoration examples. The work is done when the specification clearly defines which serialization to choose when multiple minimal representations have the same meaning.