Loads indie-config and if available, apply handlers to <indie-action> tags

indie-config.js

// Lazy-create and return an indie-config load promise
// The promise will be resolved with a config once the indie-config has been loaded
var loadIndieConfig = function () {
    
    // Create the Promise to return
    var loadPromise = new Promise(function (resolve) {
        
        // Parse the incoming messages
        var parseIndieConfig = function (message) {
            
            // Check if the message comes from the indieConfigFrame we added (or from some other frame)
            if (message.source !== indieConfigFrame.contentWindow) {
                return;
            }
            
            var indieConfig;
            
            // Try to parse the config, it can be malformed
            try {
                indieConfig = JSON.parse(message.data);
            } catch (e) {}
            
            // We're done – remove the frame and event listener
            window.removeEventListener('message', parseIndieConfig);
            indieConfigFrame.parentNode.removeChild(indieConfigFrame);
            indieConfigFrame = undefined;
            
            // And resolve the promise with the loaded indie-config
            resolve(indieConfig);
        };
        
        // Listen for messages from the added iframe and parse those messages
        window.addEventListener('message', parseIndieConfig);
        
        // Create a hidden iframe pointing to something using the web+action: protocol
        var indieConfigFrame = document.createElement('iframe');
        indieConfigFrame.src = 'web+action:load';
        document.getElementsByTagName('body')[0].appendChild(indieConfigFrame);
        indieConfigFrame.style.display = 'none';
    });
    
    // Ensure that subsequent invocations return the same promise
    loadIndieConfig = function () {
        return loadPromise;
    };
    
    return loadPromise;
};

xtag.register('indie-action', {
    lifecycle: {
        created: function() {
            if (!this.innerHTML) {
                this.innerHTML = '<a href="#"></a>';
            }
            first('a', this).textContent = this.getAttribute('do').substring(0, 1).toUpperCase() + this.getAttribute('do').substring(1);
            
        },
        inserted: function() {},
        removed: function() {},
        attributeChanged: function() {}
    },
    events: {
        'click:delegate(a)': function(e) {
            var elem = this.parentNode;
            e.preventDefault();
            var d = elem.getAttribute('do');
            var w = elem.getAttribute('with');
            var href = this.href;
            if (d && w) {
                loadIndieConfig().then(function(indieConfig) {
                    var handler = indieConfig[d];
                    if (handler) {
                        href = handler.replace('{url}', encodeURIComponent(w || window.location.href));
                    }
                    window.location = href;
                });
            }
        }
    },
    accessors: {
    },
    methods: {
    }
});

I have not quite figured out how to detect that no web+action handler is registered, and fall back to appropriate (e.g. twitter intent) urls.