To avoid clobbering the diff discussion with another commit, how about this instead?

    /**
     * The HTML of the button's label
     *
     * **IMPORTANT**
     * This attribute accepts HTML and inserts it into the DOM **without**
     * sanitization.  This attribute should only be used with HTML that has
     * either been escaped (using `Y.Escape.html`), or sanitized according to
     * the requirements of your application.
     *
     * If all you need is support for text labels, please use the `label`
     * attribute instead.
     *
     * @attribute labelHTML
     * @type HTML
     */