It seems that different people have different opinion about what setProperty should do when the property is already specified, and in particular if the existing property has its important flag set.

There are two axes of cases: third argument omitted/""/"important" and property not present/present with important unset/present with important set. That gives us the following cases to consider (using 'color' as example):

1. setProperty("color", "blue") and color is not present.
2. setProperty("color", "blue", "") and color is not present.
3. setProperty("color", "blue", "important") and color is not present.
4. setProperty("color", "blue") and color is present with important flag unset.
5. setProperty("color", "blue", "") and color is present with important flag unset.
6. setProperty("color", "blue", "important") and color is present with important flag unset.
7. setProperty("color", "blue") and color is present with important flag set.
8. setProperty("color", "blue", "") and color is present with important flag set.
9. setProperty("color", "blue", "important") and color is present with important flag set.

There are three behaviors on the table for how to handle these. For most cases the result is identical:

A, B, C:
(1) append a new declaration with the given value and important flag unset.
(2) append a new declaration with the given value and important flag unset.
(3) append a new declaration with the given value and important flag set.
(4) update the value, leave the important flag unset.
(5) update the value, leave the important flag unset.
(6) updates the value and sets the important flag.    
(7) ???
(8) ???
(9) updates the value, leaves the important flag set.

For cases (7) and (8), the proposed behaviors are:

A:
(7) no-op
(8) no-op

B.
(7) updates the value, unsets the important flag.
(8) updates the value, unsets the important flag.

C:
(7) updates the value, leaves the important flag set.
(8) updates the value, unsets the important flag.

The status in browsers and the spec is as follows:

* IE/WebKit/Blink implement A.
* Gecko/Presto implement B. (Originally Gecko implemented A.)
* The spec had B until recently but changed to C, because there was a use case for it [1] and it was argued to be saner behavior than B [2].

bz argues [3] that A is preferable since he thinks the mental model if setProperty should be "append a declaration" and let it have the same behavior as it would have declaratively.

Tab argues that C is preferable since the author just wants to change the value and usually doesn't care about the priority, especially not when omitting the priority argument.

I'd like to get some more opinions on this issue, especially from relevant developers from non-Gecko browsers.

For context, see this thread http://lists.w3.org/Archives/Public/www-style/2013Aug/thread.html#msg134

[1] http://lists.w3.org/Archives/Public/www-style/2013Aug/0168.html
[2] http://lists.w3.org/Archives/Public/www-style/2013Aug/0240.html
[3] http://lists.w3.org/Archives/Public/www-style/2013Aug/0431.html
[4] http://lists.w3.org/Archives/Public/www-style/2013Aug/0433.html

(Note that I'm not asking how to address the use case in that thread, but what setProperty should do. When we have that established we can revisit the use case and see if and how it should be addressed.)