Run this:

    document.body.style.cssText = "margin: var(--m); margin-top: 1px";
    document.body.style.cssText;

Expected: "margin-right: ; margin-bottom: ; margin-left: ; margin-top: 1px;"

Actual: "margin: var(--m);"

The problem is that StyleProperties::asTextInternal() sees that margin-right is set to a pending-substitution value for margin:var(--m), and then https://searchfox.org/wubkat/rev/7a292520f6b12e8d4d9001d1480474b5c83cb0f8/Source/WebCore/css/StyleProperties.cpp#1795 sets value to "var(--m)".

    value = substitutionValue.shorthandValue().cssText();

The value should instead be obtained in https://searchfox.org/wubkat/rev/7a292520f6b12e8d4d9001d1480474b5c83cb0f8/Source/WebCore/css/StyleProperties.cpp#1823

    value = getPropertyValue(shorthandPropertyID);

which will properly detect that "var(--m)" is not a proper serialization.