Modernizr feature-detect for css transitions on pseudos elements

css-transitionspseudos.js

// transitions pseudos test.
// check if CSS3 transitions take effect on pseudo elements.
// https://github.com/Modernizr/Modernizr/issues/295

// This test is asynchronous. Watch out.

// without csstransitions this check makes no sense and
// without getComputedStyle we wont be able to get the style from a pseudo element
if (Modernizr.csstransitions && window.getComputedStyle) {

    Modernizr.testStyles(
        '#modernizr:after{content:"";font-size:0;'+ Modernizr._prefixes.join('transition:2s;') +'}' +
        '#modernizr._m:after{font-size:999px;}',
        function(elem) {
            elem.className = '_m';

            setTimeout(function() {
                Modernizr.addTest(
                    'csstransitionspseudos',
                    window.getComputedStyle(elem, ':after').fontSize !== '999px'
                );
            }, 50);
        }
    );

}