Requests for comment/Allow styling in templates

Background
Currently the language MediaWiki uses to author documents allows the inclusion of html elements with style attributes. Mixing layout with content causes various problems with reusability, maintainability and is not future proof. Already on the mobile version of MediaWiki problems are being seen with inline styles with how pages render on smaller screens which could be solved via css3 media queries, however these cannot be done in inline styles.

It should be possible for templates to have associated stylesheets so that media queries can be applied where necessary. This is related to 35704.

A proof of concept can be found here: https://gerrit.wikimedia.org/r/68123

Problem
In terms of how separating styles into stylesheets will help things out on mobile fundamentally it allows you to use media queries. Thus you can do radically different things such as have a 2 column layout on a desktop browser but have a 1 column layout on mobile. For instance in this code snippet:

This means if the screen is smaller than 600px then none of these css rules will apply. On a screen of 320px such a style introduced via inline style would be broken ( and you cannot do media queries in a style attribute )

Various rendering issues on mobile are caused by the existence of inline styles which do not degrade. See bug 35704.

Challenges

 * Templates could use conflicting CSS names causing incompatibilities between templates
 * This is seen as a good thing as it leads to better template maintenance and promotes reuse and collaboration
 * CSS rules within templates could be applied to UI chrome
 * This can be resolved by using scoped style tags that only apply styles the template itself - scoped styles is currently a W3C candidate recommendation
 * For older browsers that do not support scoped CSS there is a JavaScript shim
 * For browsers that do not support JavaScript there is a danger that CSS could leak outside templates. It is decided that this in acceptable tradeoff
 * with LESS support and some work we could fallback to scoping the stylesheet to #bodyContent to at least prevent spillage into the UI
 * Balanced and unbalanced templates could lead to styles leaking to places that they shouldn't
 * It is proposed that by allowing template designers to add style tags in the wikitext itself this gives them control to ensure they are applied correctly.
 * What about templates that are used multiple times in a page?
 * This can lead to CSS duplication.
 * It is suggested that a template that is used multiple times on a page probably should still make use of inline styles or Common.css so this might not be a problem that should be worried about.

Implementation details

 * If a style tag is not inserted with the scoped attribute the styles do not get applied to the page and shows up HTML-encoded (the same as any HTML tag that is not allowed).

Examples

 * Template:User_info
 * Usage: User:SueGardner could benefit from media queries to turn a 3 column layout into a single column layout