In section 9 of the current spec, Rendering HTML Templates, the rendering of the template element itself is defined as hosting an empty shadow tree. 

Rather than taking a dependency on Shadow DOM, this could be defined more simply as an addition to the default style sheet. This is also more consistent with the behavior of other 'hidden' elements in HTML (see http://dev.w3.org/html5/spec/rendering.html#hidden-elements):

    @namespace url(http://www.w3.org/1999/xhtml);
    template {
        display : none;
    }