Zen Templates is based on an idea I've been tossing around for about six months. It started with a frustration that there was no way to validate a page written in PHP or JSP as valid HTML without executing it to get the output. It seemed like there had to be a way to accomplish that.
I started out looking into what I knew were global attributes, class and id. I did some research and found that the standard allows any character in a class or id; this includes parens and such, meaning a functional syntax could be used in these attributes, which a parser could then process to render the template.
This seemed practically ideal; I could inject content directly into the document, identifying the injection targets using these custom classes. I toyed with the idea of using this exclusively, but saw a couple of serious shortcomings. For one, sometimes you want to insert dynamic data into element attributes, and I didn't see a good way to handle that without allowing a substitution syntax like that of JSP or ASP. I decided this would be a requirement to do any real work with it.
I also saw the problem of templates. Often each page in a dynamic site is called a template, but I'm referring to the global templates that all pages on a site share, so there is only one place to update the global footer, for example. I had no good solution for this. I started thinking about the idea of each page being a template and sharing a global template - much akin to subclasses in object oriented programming, one template derives from another.
I started batting around different possibilities for deriving one template from another, and decided on having a function (in a class attribute) to identify the template being derived from, with hooks in the parent template to indicate which parts the "subtemplate" would be expected/allowed to "override".
I let the idea percolate for a while - a few weeks - as other things in life kept me too busy to work on it. Eventually it occurred to me that all these special functions in class attributes were pretty messy, and a poor abstraction for designers. It could potentially interfere with styling. It would produce ugly code. And I was on a semantic markup kick, and it seemed like a perfect opportunity to do something useful with semantic markup.
So I started rebuilding the concept and the current Zen Templates was born (and the name changed from its original, Tabula Obscura.) As I committed to maximizing the use of semantic markup and keeping template files as valid, usable HTML, I reworked old ideas and everything started falling into place. I remembered that the new HTML5 data attributes are global attributes as well, and would give me an additional option for adding data to markup without interfering with classes or ruining the semantics of the document.
I ironed out all the details of how derivation should work; it made semantic sense that a page that derived from another page could be identified by class; and, taking a page from OOP's book, it made sense that an element in the subpage with the same ID as an element in the parent page would override that element, making any element with an ID automatically a hook; somewhat like a subclass overriding methods in the superclass by defining a method with the same signature.
I sorted out the details of content Injection as well, thinking that, semantically, it made sense that an element of a certain class would accept data from the model with an identifier matching the class name. Even better, I didn't need a looping syntax; if you try to inject a list of items into an element, it would simply repeat the element for each item in the list. This simplified a lot of syntax I've had to use in the past using JSP or Smarty.
I also wrote out how substitution should work, using a syntax derived from JSP. Leaning on JSP allowed me to answer a lot of little questions easily. I would try to avoid the use of functions in the substitution syntax, because it does make the document messier, and forces more programming tasks on the designer. I conceded that some functions would likely be unavoidable.
When I felt like I had most of the details ironed out, a guiding principal in mind, and a set of rules of thumb to help guide me through questions down the road, I was ready for a prototype. Stay tuned for Part 1!