MediaWiki:Mobile.css was added to MobileFrontend as a replacement for MediaWiki:Common.css
Many projects use MediaWiki:Mobile.css. However many of the rules there are redundant and necessary on mobile domain given the mobile HTML is different from desktop and many of these wiki pages result from copying and pasting from Common.css. Mobile also provides opinionated rules on how horizontal lists should render. Many of the rules only apply to a handful of pages. For instance the hlist template is not needed on all pages yet CSS for it is loaded everywhere.
As a workaround (which quite frankly is a bad workaround) we load this CSS via JavaScript (See T190083)
Now that TemplateStyles exists there is no reason to use MediaWiki:Mobile.css other than 1) convenience (e.g. it's easier to add a class `hlist` to an element then to find and use a template and 2) style the UI e.g. change the header color - Minerva doesn't want support this right now and if we do want to support that better more scoped solutions exist.
Gadgets provide a better solution for scripting than Mobile.js
= Proposal =
Proposal is as follows
[] Blocking problems in TemplateStyles that are blocking wider use of template styles are fixed T162379
[] Community is notified and supported with moving styles to appropriate stylesheets shipped with TemplateStyles.
[] MediaWiki:Mobile.css and Mobile.js is removed from the site. MediaWiki:Common.css and Common.js will not be loaded in their place.