From validator.w3.org - August 28, 2013 2:49 AM

This checker performs various tests on a Web Page to determine its level of mobile-friendliness. The tests are defined in the mobileOK Basic Tests 1.0 specification. A Web Page is mobileOK when it passes all the tests. Please refer to the About page for more details. If you wish to validate specific content such as markup validitation, or RSS/Atom feeds, or CSS stylesheets, or to find broken links, there are other validators and tools available.