Adam Ahmed

Yeah I'm with @gma. I'm pretty sure what you really should be doing if you want to make page transitions pretty is not actually do page transitions at all. The History API is for exactly this purpose.

Something like (but not exactly - this is untested):

a.onclick = function(e) {
    e.preventDefault();
    fadeOut().then(function() {
        history.pushState(null, null, a.href);
        load(a.href).then(fadeIn);
    });
};
window.onpopstate = function() {
    fadeOut.then(loadContent).then(fadeIn);
}