[cssom] shorthand not representing the values of list in its grammar?

[waldbaerkoch]

6.7.2. Serializing CSS Values
Step 1.2:

If shorthand cannot represent the values of list in its grammar, return the empty string and terminate these steps.

This step is invoked if the procedure is called with a list of longhand declarations:

• step 3.2.5 of 'serialize a CSS declaration block'
• step 1.2.3 of getPropertyValue(property)

In which situation can the shorthand not represent the values of list in its grammar? That sounds like a declaration passed to this procedure can have an invalid value. As I read the procedures for manipulating a CSSStyleDeclaration the values are always checked against spec:

• setting cssText: in step 3 calls 'parse a CSS declaration block' which parses "declaration according to the appropriate CSS specifications, dropping parts that are said to be ignored".
• setProperty(property, value, priority): in step 5 calls 'parse a CSS value', which matches "list against the grammar for the property property in the CSS specification", and then in step 6 terminates if the parsing returns null.
• setPropertyValue(property, value): in step 4 calls 'parse a CSS value', and then in step 5 terminates if the parsing returns null.

[Loirooriol]

I guess it's e.g.

el.style.border = "1px solid blue";
el.style.border; // "1px solid blue";
el.style.borderTopColor = "red";
el.style.border; // ""

[tabatkins]

Yeah, there are a number of situations where you can set up the longhands in such a way that they can't be represented by the shorthand. Nothing invalid here, just things getting more complex than the shorthand can handle, like @Loirooriol illustrated.