Bing is following Google's lead with mobile friendly tags and algorithm shift.
Bing's Factors determining mobile friendliness
Bing consider a number of different factors in making this determination. Some of the more important factors are detailed below:
1. Navigation – The menus, buttons and links on the page should be large enough and spaced well apart to aid touch-based navigation. You can see the large, easy-to-tap and distinct navigational elements on the mobile-friendly page. In contrast, the no mobile friendly page has hyperlinks that are too close to each other and could be prone to accidental clicks from mobile users.
2. Readability – The text on the page should be readable without requiring zooming and lateral scrolling to access specific content. Keep in mind that readability is influenced both by font size and the viewport settings (defined in HTML tags).
3. Scrolling – The content of the web-page should fit within the device width. Vertical scrolling is considered acceptable, but the need to scroll horizontally hampers the ability to consume your content easily. Well-designed mobile pages typically fit well to the device width in both portrait and landscape orientations.
4. Compatibility – The content needs to be compatible with the device. For example, pages with flash content do not work well on iOS devices This also applies to videos that cannot be played on mobile devices due to plugin dependencies, copyright issues or distribution decisions made by the content owner. Bing checks for hints in the content rendered on the page to determine if any compatibility issues exist.
Typically all the factors above will need to be met for a webpage to be considered mobile-friendly by Bing's classification algorithm. There are more factors that Bing is considering along the lines of mobile friendliness, like the friction that pop-ups sometimes create in navigating to the core content of the page. One important thing to remember – Bing depends on access to all the necessary CSS and script files required by your page to make this determination. So, it is important that you allow Bingbot mobile user agents access to download these resources.