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I want to define page layout templates - not the current Templates that are more
like macros, but rathera common page layout that I can associate with pages, for
example with a new namespace "Layout:",

  [[Layout:MyLayoutName]]

or perhaps indirectly by associating a layout with a category, or as a choice in
the page editor.

For example, I may want to automatically include a header, navigation, and
footer template on a subset of pages, for example, for articles about Eclipse. I
may define a layout, called [[Layout:Eclipse]], as follows

   {{Eclipse-heading}}
   {{Page:content}}
   {{Eclipse-navigation}}
   {{Eclipse-footer}}

However, I don't want to have to include these templates in each and every page
in the Eclipse category.
Rather, I would like to create a [[Layout:Eclipse]] page layout and by
referencing [[Layout:Eclipse]] on these pages (similar to how one references
categories), it will use that page layout; i.e. automatically include the
heading, navigation, and footer. Such layouts may contain <div> or <table> or
other layout markup, whether (restricted) html or wiki.
(I'm suggesting one implementation; other implementations for assiging a page
layout to
a page are also possible.)

When a page is previewed or saved, its body is substituted where
{{Page:content}} (or some other meaningful marker/variable) appears, and the
other templates/markup inserted as if they were on the original page. User's
editing such pages would not have to explicitly include this boilerplate, and I
could change this page layout template once and all pages which use it would
render with the new layout.

Only one page layout is needed per page, although I can see how it may be
possible/desirable to nest them via a Decorator pattern; the output of applying
one page template then becomes the {{Page:content}} for the next. This would be
tricky; you would not want to reformat/reinterpret the text as markup. In
addition, some layouts may themselves reference other layouts. (Beware of
cycles, though!)

Finally, I would expect to have a default page layout for the site; i.e.
[[Layout:default]] or [[Layout:site]]. For example, there is an implicit page
layout template already which includes MediaWiki:Sitenotice and
MediaWiki:Tagline. I would like to make it more explicit and customizable and
allow different pages or categories to have their own custom page layouts
(either replacing or augmenting other layouts).