Microsoft today announced the general availability of the Bing Mobile Friendliness Test Tool, a simple service that can analyze a website and tell you if it’s mobile-friendly based on the Microsoft Bing search engine’s standards and, if it’s not, what’s wrong with it.

Bing began marking websites as mobile friendly in search results earlier in the year. The designation affects the ranking of websites in mobile search results, so this is important, especially as the Web shifts to being mobile-first.

Google started tagging pages as mobile friendly in search results and changing search ranking accordingly earlier this year, right alongside Bing.

Bing’s system takes several factors into consideration, including text readability, link size, and the width of page content based on the size of a mobile device. Responsive design is the usual term for that last criterion, and more websites have been adopting it in the past couple of years.

Another interesting element of design the tool checks is something called zoom control. Basically, the ability to zoom is good, and the inability to zoom is bad.

“The Zoom Control check verifies if the configuration of the viewport hampers the user’s ability to pinch and zoom the page,” Microsoft senior program manager of webmaster tools Charu Puhazholi and mobile ranking program manager Shyam Jayasankar wrote in a blog post on the Mobile Friendliness Test Tool. “In general, not using the scale-related viewport settings should result in your page being zoomable on most mobile browsers, but improper use of these settings (user-scalable, maximum-scale, minimum-scale) could result in hampering access to some content on the page. Some mobile-friendly pages prevent user zoom by design and we do take that into account before flagging it as an error.”

Incompatible plugins like Flash are also looked down upon.

Check out the full blog post from Puhazholi and Jayasankar to learn more about the most important signals Bing reviews to rank sites in mobile search results.