Currently the MediaWiki markup 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 T37704.
This RFC was proposed in 2013, discussed at two separate RFC meetings in 2014, and the status is unclear as of 2016-03-29. More detailed status in the comments of this task
Allow styling in templates RfC.