Microsoft Bing has now started tagging search results as “Mobile-friendly” for websites which are mobile optimized. This enables the user to skim through the search results to know which ones will quickly answer their information needs. Unlike Google, Microsoft won’t penalize websites for not having mobile friendly sites in the search results.
Our approach to mobile friendliness as a ranking signal balances the need to improve the ranking for mobile-friendly pages, with the continued focus on delivering the most relevant results for a given query. This means that for mobile searches on Bing, you can always expect to see the most relevant results for a search query ranked higher, even if some of them are not mobile-friendly. While the changes will improve ranking for mobile-friendly pages, webpages that are highly relevant to the given query that are not yet mobile-friendly will not get penalized. This is a fine balance and getting it right took a few iterations, but we believe we are now close. We expect to start rolling out mobile friendliness ranking changes in the coming months.
Bing considers a number of different factors in making mobile friendliness determination. Read about it here.