Parsoid is trying to encourage wikis to rely on CSS for customizing Cite output instead of site messages. Parsoid's Cite output currently doesn't look at wiki's site messages. Since Parsoid output is used in VisualEditor, one way for wikis to identifying CSS styles that needed to be added is for them to look at Cite output in VE and add CSS styles that makes the output look similar to how their read html (non-Parsoid) currently looks.
These CSS styles would be added to the wiki's common.css (or wherever these wiki-specific styles reside).
However, this should be done after T156350: Add language-specific CSS modules for Parsoid's Cite output is done.