sanitized-css content (the CSS stylesheets used by TemplateStyles) can only be created in the Template namespace, with a .css extension. But there are ways other titles can end up with such content:
- create as wikitext, change content model via Special:ChangeContentModel (needs admin rights)
- create in Template namespace then move (anyone can do it)
- create via the API and set the contentmodel parameter (anyone can do it)
Given that even most admins don't understand the concept of content models, this has a high potential for confusing editors. At the same time there does not seem much use for it; should we just restrict sanitized-css to Template:*.css?
(A somewhat related issue is that moving pages without leaving a redirect is normally an admin-level feature, but it will happen for everyone with template styles because the default content model is not wikitext so a redirect cannot be created. That's again bound to confuse people.)