As I write this, media queries are available in Internet Explorer 9 and above and in all other major modern browsers. The media features related to device dimensions are the most widely implemented. The resolution media feature is in Internet Explorer 10, Firefox, and Opera and was implemented in the WebKit core at the end of 2012, so it’s making its way into WebKit-based browsers.
The dppx unit should be in all modern browsers bar Internet Explorer by the time you read this, and devicePixelRatio is in WebKit browsers, Opera, and Firefox (including mobile versions).
The @viewport at-rule is in Opera, Internet Explorer 10, and WebKit, using the vendor prefix of each. The matchMedia API is in Internet Explorer 10 and all other modern browsers but not in Android 2.3 and below.
The CSS property box-sizing is in all browsers, although it requires a vendor prefix in Firefox and versions 3.0 and below of Android. Only Firefox supports the padding-box value. The calc() value function is in IE9 and above, Firefox, desktop WebKit browsers, and from version 6.0 of mobile Safari. It’s not in Android or Opera and requires the -webkit- prefix in other mobile WebKit browsers.
The viewport-relative length units—vh, vw, etc.—are in IE9 (with a few bugs) and IE10, Firefox, and most WebKit browsers except Android, but not present in Opera. The rem unit is in IE9 and above and all other major browsers.
The object-fit and object-position properties are implemented in Opera only and marked as “at risk” in the spec, so face an uncertain future, especially now that Opera is moving to use WebKit.