带有BASIC whiteListType的SafeHtml在与rel = nofollow的链接上失败

Question

假设，我有一些字段，我想只允许一些基本的HTML，如粗体或段落甚至链接：

@SafeHtml(whitelistType = SafeHtml.WhiteListType.BASIC, message = Constants.HTML_IS_NOT_ALLOWED)
private String body;

hibernate文档接下来说明基本白名单：

    /**
     * This whitelist allows a fuller range of text nodes:
     * <code>a, b, blockquote, br, cite, code, dd, dl, dt, em, i, li, ol, p, pre, q, small, strike, strong, sub, sup, u, ul</code>
     * , and appropriate attributes.
     * <p/>
     * Links (<code>a</code> elements) can point to <code>http, https, ftp, mailto</code>, and have an enforced
     * <code>rel=nofollow</code> attribute.
     * <p/>
     * Does not allow images.
     */

根据它，验证器应允许链接nofollow属性，例如

<a href="http://stackoverflow.com/" rel="nofollow">stackoverflow</a>

但验证仍然失败，为什么？