The tool evaluates a page to check if it is mobile-friendly based on these criteria:
- Viewport and Zoom control configuration
- Width of page content
- Readability of text on the page
- Spacing of links and other elements on the page
- Use of incompatible plug-ins.
Here is a screen shot of the tool in action. As you can see, it shows a preview of our site, www.infuga.ro, on the right side:
Google launched their mobile-friendly testing tool just about a year ago today. Bing has not yet announced the launch of their mobile-friendly algorithm; that may come soon.