IMO the Vector skin in core should not contain selectors for things added by extensions, such as #centralNotice.
#centralNotice
Good point, I've moved the Central Notice styling from the skin to the extension as of r67928.
Hmm, you are deprecating 1 CSS id, and adding 2 new CSS ids. I think just adding #centralnotice, and keeping #sitenotice should be considered. Deprecating ids without good reason usually just breaks scripts.
"#sitenotice" is being kept. The ids that are being deprecated are "#sitenotice div" and "#sitenotice p" (neither of which are defined in all the skins). These unspecific sub-selectors were causing unintended styling issues to central notice banners (which are also included in the sitenotice div along with local sitenotices). To resolve this, I created specific divs to identify central notices and local notices within the sitenotice div and now apply specific styling to each. In this way, we can style local sitenotices without also messing up the styling of central notices and vice versa. I retained the original #sitenotice container div for exactly the reason you are suggesting (to not break things), even though it is largely unnecessary now that there are more specific divs within it.
If any IDs / classes are being added, shouldn't they have a "mw-" prefix?
To give you a better idea, here's what the HTML looks like: <div id="sitenotice"> <div id="centralNotice">...</div> <div id="localNotice">...</div> </div>
Now, all 3 of these can be styled specifically, and we aren't using generic selectors that apply (unintentionally) to every p and div in the banners.
@MZMcBride: #centralNotice has already been in use (by the extension) and isn't a new addition. I would use #mw-localNotice, but it would seem odd since we're using #siteNotice and #centralNotice for the other pieces. Perhaps we could change them all at some point, although this would necessitate updating a hundred or so centralNotice banners. Which is worse: Having the names be inconstant with each other or inconstant with the convention?
The issue is that IDs in the skin can conflict with the IDs used to link to a specific section inside wikitext. But #localNotice will do for now.