Some protected, complex, or well-tested templates have a sandbox version for testing before making changes that will affect live articles. With the introduction of TemplateStyles, that also means live and sandbox versions of the stylesheet.
Some of these templates also have "test case" pages that display side-by-side comparisons of the output of the live and sandbox versions of the template so rendering changes can be more easily observed. But since the live and sandbox versions use the same selectors, this fails to accurately show the differences since both wind up being affected by both stylesheets.
This one came up in conversation at en.WP at WT:TemplateStyles a bit of the way down the section.
There may not be a good technical solution to this, but I figured I'd get it on the workboard and see if there is one at all. :)
(Keep in mind MCR I guess?)