Based on the current W3 Web Components draft, X-Tag enables the custom element portion of the proposal. Custom elements let you register new tags/elements with the parser so they are recognized and inflated with special abilities you define.
Find out more about what X-Tag does, where it works, and how to use it, on the project page here: http://mozilla.github.com/x-tag/
X-Tag (excluding third-party icons or images) is subject to the terms of the Mozilla Public License, v. 2.0. If a copy of the MPL was not distributed with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
To get started hacking on X-Tag, first check out the code and be sure you also clone the elements submodule:
git clone --recursive https://github.com/mozilla/x-tag
Open up demo.html in your browser to see how things work.