Rijk was kind enough to point out that my offline version of the CSS2.1 sidebar is out of date.
Rijk’s Panelizer is a collection of Sidebars for Opera (may work in Firefox too, but since I don’t use it, I can’t say for sure).
Most notable in the collection (for me at least) is an updated version of the CSS2.1 Sidebar which points to the CSS2.1 Candidate Recommendation (CR) rather than the older and outdated Working Draft.
Rijk has been nice enough to provide a zip file you can download and use offline.
See also: Rijk’s Tweak Opera page and other sidebars here at TnTLuoma.