Mobile preferred

Mobile optimised websites are marked with an AMP icon in the Google search listing starting Februari 24, 2016. This icon highlights results as Accelerated Mobile Pages to provide users with some feed forward on the mobile experience they will encounter when clicking a search result link. The link is shown dynamically (only on mobile). As of July 2018 Google decided to not only add an icon to mark positive mobile experiences, it includes mobile performance as a criteria for the ranking algorithm of the search engine.

AMP IconGoogle launched a couple of websites and services to guide developers in creating websites that are "Mobile friendly". They let you test a website on it's mobile friendliness and they have a Mobile playbook that showcases some fresh ideas on how to design the mobile experience. Google now recommends Responsive Web Design or RWD as the way to go. Gooo Google!
Google search results on Mobile with the AMP indication