says : none | [ all | <IDENT> ] [ ‘,’ [ all | <IDENT> ] ]*
so it is possible to have "background, font-size, all" as a value. I'm not sure what should be the behavior here : animating all the properties with the last index in the lists of "transition-duration", "transition-delay", ... or animating background, font-size with their respective indices in the lists of "transition-duration", "transition-delay", ... and animating all the other properties with the last index in the lists of "transition-duration", "transition-delay", ...
In the other hand "background, font-size, all" goes in conflict with "Otherwise, a list of properties to be transitioned, or the keyword ‘all’ which indicates that all properties are to be transitioned, is given." which seem to assume that all comes alone.
Side note : Opera reject this value.
The spec is somewhat badly worded. Treat 'all' like an omni-shorthand for every property, and then it just acts the same as every other shorthand.
(This is in fact literally true now, since we added the 'all' property <http://dev.w3.org/csswg/css3-cascade/#all-shorthand>. The name was inspired by the Transitions value. ^_^)
<TabAtkins> "<anything>, all" should be equivalent to just "all", and "all, <anything>" should animate everything but the <anything> with the first transition, and the <anything> with the second.
Before closing the bug, or make a patch, I will investigate this behavior.
I double checked and we do follow the behaviour described by Tab.
http://jsbin.com/upafut/3/edit for the record.