WebComponentsReady is an event in the Polymer stack that fires when all known registered custom elements are ready for interaction - X-Tag has similar event (now just a hook into the polyfill's) called DOMComponentsLoaded. Being notified that all elements in source are ready for interaction is essential at any point in the page lifecycle when a new element is defined.
This is not an X-Tag issue, it's a Web Components issue - one we need to address.
