There are 2 types of "JSP" you might be referring to here.
One is the traditional form of JSP, that does not use JSF.
The other is the JSP form that served as a View Template in JSF version 1.
In JSF version 2, the second form of JSP is no longer supported. In JSF2, the View Template Language is xhtml format. Which means that regardless of what else you do,
you should be coding xhtml for anything that your expect to keep working as JSF1 support dies. And since JSF2 has been the standard for several years now, JSF1 support is already not very widespread.
Traditional JSPs are still supported, as are traditional
servlets, but they are not processed by the FacesServlet and therefore cannot use JSF tags or JSF-mode objects such as the FacesContext.
The best way to keep a set of websites consistent as far as appearance goes is to employ a standard set of CSS classes. That way, you can make xhtml, JSP and HTML output all look consistent. In the case where you have multiple apps all using the same CSS, it may actually be better to have a master webapp that's independent of the other webapps whose sole purpose is to contain CSS URLs. Since all it would contain is fixed content, it could even be a non-J2EE app, if your are using a frontend server such as Apache.