Load a TypeKit Font Kit inside of the TinyMCE Visual Editor in WordPress.

typekit.js

(function() {
    tinymce.create('tinymce.plugins.typekit', {
        init: function(ed, url) {
            ed.onPreInit.add(function(ed) {
                // Get the iframe.
                var doc = ed.getDoc();

                // Create the script to inject into the header asynchronously.
                var jscript = "(function() {
                    var config = {
                        kitId: 'gcc4mmp' // YOUR TYPEKIT ID GOES HERE INSIDE THE SINGLE QUOTES.
                    };
                    var d     = false,
                        tk    = document.createElement('script');
                    tk.src    = '//use.typekit.net/' + config.kitId + '.js';
                    tk.type   = 'text/javascript';
                    tk.async  = 'true';
                    tk.onload = tk.onreadystatechange = function() {
                        var rs = this.readyState;
                        if (d || rs && rs != 'complete' && rs != 'loaded') return;
                        d = true;
                        try { Typekit.load(config); } catch (e) {}
                    };
                    var s = document.getElementsByTagName('script')[0];
                    s.parentNode.insertBefore(tk, s);
                })();";

                // Create a DOM script element and insert the code inside of it.
                var script  = doc.createElement("script");
                script.type = "text/javascript";
                script.appendChild(doc.createTextNode(jscript));

                // Append the srcript to the header.
                doc.getElementsByTagName('head')[0].appendChild(script);
            });
        },
        getInfo: function() {
            return {
                longname:  'TypeKit',
                author:    'Thomas Griffin',
                authorurl: 'https://thomasgriffin.io',
                infourl:   'https://twitter.com/jthomasgriffin',
                version:   '1.0'
            };
        }
    });
    tinymce.PluginManager.add('typekit', tinymce.plugins.typekit);
})();

You just solved my biggest issue ever occured! Big thanks 👍