Web frameworks are increasingly adding support for multiple templating engines, because often you already have a lot of templates and all you need is just faster processing on the backend. Here is a compilation of Template Loader API from various engines, which may serve as a common ground for interchangeable templating engine interface:
Django - https://docs.djangoproject.com/en/1.5/ref/templates/api/#using-an-alternative-template-language
Genshi - http://genshi.edgewall.org/wiki/Documentation/0.6.x/loader.html