CSS Media Queries is a module that allows you to test and query the values or features of the user agent or display device.
«Media Queries enable CSS to query various aspects of the page's environment, including things that can be difficult or impossible to find via scripting. This is potentially a privacy hazard, allowing enhanced fingerprinting of a user, but the risk is generally low. At a minimum, the same information should be inferrable via scripting by examining the User-Agent string. However, UA string spoofing does not affect Media Queries, making this a somewhat more robust detection technique.»