[Webkit-unassigned] [Bug 225206] New: CSS shorthand should serialize as empty string when longhands have variables

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=225206

            Bug ID: 225206
           Summary: CSS shorthand should serialize as empty string when
                    longhands have variables
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: obrufau at igalia.com

Run this code:

```js
document.body.style.cssText = `
  --t: 1px 2px;
  --r: 3px;
  --b: 4px;
  --l: ;
  margin-top: var(--t);
  margin-right: var(--r);
  margin-bottom: var(--b);
  margin-left: var(--l);
`;
getComputedStyle(document.body).margin; // "0px 3px 4px 0px"
document.body.style.margin; // BAD: "var(--t) var(--r) var(--b) var(--l)"
document.body.style.margin += "";
getComputedStyle(document.body).margin; // "1px 2px 3px 4px"
```

The margin shorthand shouldn't be serialized concatenating the var() notations, because that can have a different meaning!

It should be serialized as empty string instead.

The specs are not much clear about this (https://github.com/w3c/csswg-drafts/issues/6253), but Chromium and Firefox do the reasonable thing and serialize as empty string.

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