I'm excited to see Kevin's work on template plugins, and that the TurboGears plugin system is fully extracted into a generic plugin for templates. I think this will be an easy ad hoc standard to start supporting elsewhere. It should encompass all the "normal" templating languages that just take a bunch of variables (something like meld3 probably won't work with this, though). I doubt code can be very well neutral with respect to the template language it is feeding. Kid and Cheetah can have arbitrary Python to munge the data in them. ZPT can have Python expressions, but not statements or functions. Django and ClearSilver need data structures that require very little processing in the template, as they don't have generic expressions at all. So you can't exactly be neutral. But still this is an important step forward.

One of the big issues in making little apps is how to make them fit into a larger site. It's mostly fine if the internals of the pages are written by different templating languages. Standard conventions for CSS classes would be a tremendous boon, but a little sloppiness isn't that big a deal. But you still need an application that is well-wrapped in some template for your site. Each application may have a way to give a site template, but if they are all using different templating languages that means a lot of duplication. (And even if they are using the same templating language, some standard way to override templates is still needed, or else you have a mess of things to manage).

So... what to do...

I like HTTP, so I naturally go that direction for interoperability. Server-side includes provide a simple, relatively language-neutral way to handle templating. But they work textually, not based on markup, and there's lots of issues with the typical implementations. XSL applies well here, of course. I haven't used XSL for years, but my brief experiments with it left me so disheartened I simply can't imagine it being useful for templating.

But more primitive templating is quite possible. One of the interesting ideas I've seen (and I'm sure there's others -- if you have one, then do comment) is HTML overlays. This can be implemented on the client-side in Javascript, but of course it could be implemented as a filter easily enough as well.

Then applications would each take a site_template configuration parameter, and maybe a Javascript parameter if client-side overlays were going to be used (presumably any filter would simply notice the overlay and implement it). Then the application would put this in the head:

<link rel="overlay" type="text/x-im-not-sure" 
 href="/configured-template-location" />

Actually I now think the type should be configurable too, allowing for other kinds of templates than just this implementation of overlays. Also overlays don't seem to have a way to pull in content, they only allow content to be pushed into them.

Obviously the Javascript can be applied to any site. A server-side filter could also be easily(ish) written in C as an Apache filter, or as libraries in a variety of frameworks and languages.