Is the `extends` property deprecated in the X-Tag?

The use case for the extends property seems very straightforward ( http://www.x-tags.org/docs#custom-tag-registration-extends ), however testing with the following tag definition:

(function () {
    xtag.register('dk-foo', {
        extends: 'b',
        lifecycle: {
            created: function () {
                this.innerHTML = '*FOO*';
            }
        }
    });
}());

and markup:

<dk-foo>Hello BAR</dk-foo>

there is no effect (i.e. the text is not in bold), and, even worse, it breaks in Chrome.

I tested IE11, FF28, Safari 5.1.17 and Chrome 33/35. Each browser, except Chrome, runs the code in lifecycle.created(i.e., changes the text to *FOO*). If I remove the property extends, it will also run in Chrome.

I was no longer able to find documentation on extendsthan the above document, as well as tags that use it (although I, of course, did not look at all of them ...).

, extends..?