In the inspector, you can easily edit the shorthand for margins, padding, backgrounds, etc, but the individual properties listed when you expand the disclosure triangle are not editable.
Created attachment 41920 [details]
Demonstrates that background shorthand property is missing background-position-x and background-position-y sub-properties.
In addition, the shorthand does not show the background-position-x or background-position-y sub-properties, per the attached HTML demo.
Inspect the green div with ID "InspectMe", then double-click the background shorthand property. It will read:
background: url(http://a3.twimg.com/profile_background_images/3703201/d-a-v-e-bg.png) no-repeat scroll green;
background: url(http://a3.twimg.com/profile_background_images/3703201/d-a-v-e-bg.png) no-repeat scroll green top right;
Appears to have been resolved in some recent nightly build: Inspecting attachment above under r50006 shows and allows the following:
background: green url(http://a3.twimg.com/profile_background_images/3703201/d-a-v-e-bg.png) no-repeat scroll 100% 0%;
Editing the background-position-x and background-position-y sub-properties in the shorthand format works as expected. Original bug (that individual sub-properties are not editable when the disclosure triangle is expanded) remains.
Man, I really screwed this up: I should have filed a separate bug.
The original bug (which seems more of an enhancement request) remains: expanding a shorthand property reveals _un-editable_ sub-properties.