There are several messages which are essentially content placeholders for wiki admins to fill (e.g. the various -top and -bottom messages) and they will expect TemplateStyles to work there. It should be clearly documented somewhere whose responsibility is it to make that work - is it template authors' job to wrap the templates used in such messages in <div class="mw-parser-output">...</div> or should developers add that wrapper to code which inserts such placeholder messages?