[cssom][css-grid] Do shorthands serialize with the resolved value of their longhands?

[Loirooriol]

Let's say that the longhands of a shorthand have a resolved value which is different than their computed value.

How should the shorthand serialize? Some examples:

• For position: relative; inset: auto, all browsers serialize top as 0px. But Firefox serializes inset as auto, while Blink and WebKit say 0px.
• For margin: auto, all browsers serialize margin-top as 0px. But Firefox serializes margin as auto, while Blink and WebKit say 0px.
• For display: grid; grid-template-rows: repeat(2, 1px), all browsers serialize grid-template-rows as 1px 1px. But Firefox and Blink serialize grid-template as repeat(2, 1px) / none, while WebKit says 1px 1px / none.
• For display: grid; grid-template: 1px / 2px and ::before { content: ""; grid-row-end: 1 }, all browsers serialize grid-template-rows as 0px 1px. But Firefox and Blink serialize grid-template as 1px / 2px, while WebKit says 0px 1px / 2px.
• For display: grid; grid-template: 1px / none; width: 100px and ::before { content: ""; grid-row-end: 1 }, all browsers serialize grid-template-rows as 0px 1px and grid-template-columns as 100px. But Firefox serializes grid-template as 1px / none, Blink as 1px / 100px, and WebKit as 0px 1px / 100px.

Grid is especially nonsensical.

I tend to agree with WebKit: shorthands should serialize as if the computed value of the longhands was their resolved value.