Code for testing css-transition support on pseudo-elements for Modernizr. Forked from diseltravis's Gist: https://gist.github.com/dieseltravis/3908275 With improvements borrowed from Ryan Seddon's JSFiddle: http://jsfiddle.net/ryanseddon/vLdUq/2/

Modernizr Pseudo-element CSS-Transition test .js

(function(Modernizr) {
  var win = window,
      transitionTo = "999px";

  function _finish(result) {
    Modernizr.addTest("csstransitionspseudos", result);
  }

  if (Modernizr.csstransitions && win.getComputedStyle) {
    Modernizr.testStyles(
      "#modernizr:after{content:'';font-size:0;"+Modernizr._prefixes.join("transition:2s linear;")+"}"+
      "#modernizr.m:after{font-size:"+transitionTo+";}",
      function(el) {
        document.body.appendChild(el);

        // Get the iniital computed style to force a browser style calc
        win.getComputedStyle(el, ":after").fontSize;

        el.className += "m";

        try {
          _finish(win.getComputedStyle(el, ":after").fontSize !== transitionTo);
          el.parentNode.removeChild(el);
        } catch (e) {}
      }
    );
  } else {
    _finish(false);
  }
})(Modernizr);