Requests for comment/HTML templating library

Currently several MediaWiki extensions make use of JavaScript or PHP HTML templating libraries. It would be ideal to standardize on one library and add it into MediaWiki core. This is similar to the situation that existed regarding CSS languages before LESS support was added to core and made the standard.

Justification

 * Better code readability, editability, and portability
 * Separate code from markup
 * Avoid jQuery spaghetti
 * Avoid loading multiple JS libraries from different extensions

Existing implementations in MediaWiki extensions

 * PageTriage uses Underscore.js
 * MobileFrontend uses Hogan.js (Twitter's implementation of Mustache.js)
 * DonationInterface uses a custom template system (RapidHTML)
 * ArticleCreationHelp uses a custom template system
 * Wikibase uses a custom template system

Templating library options and requirements
It would be best if we could choose a library that has both JS and PHP implementations. That will make it easier to share/port code between client-side and server-side, and eliminate the need to learn two different syntaxes. It should also be a library that is lightweight enough to use on mobile, but flexible enough to meet the needs of diverse applications.

Requirements

 * Both a PHP and JS implementation
 * if/else
 * foreach (array handling)
 * ability to control HTML escaping (preferably with escaped output as the default)
 * Extensible enough to accommodate MW i18n system
 * No huge client-side footprint
 * Dumb (the less logic the better). Some template languages, for example jinja2 have a concept of filters where computation happens in the template itself. In my opinion this is unnecessary and can lead to complicated unreadable templates. Data should be preprocessed in PHP before being passed to a template.
 * Readable - templates should be easy to grasp for someone with basic HTML knowledge.
 * Commenting

Nice to have

 * Small client-side footprint

Implementation
On the client-side, the library would be packaged into a ResourceLoader module targeting both desktop and mobile (but not loaded by default). On the server side, we would simply include the class files in /includes/libs/ (after a security review).

Example of client-side use
Add templates to a ResourceLoader module:

Use them in JavaScript:

Example template file using Mustache syntax: