备忘下，关于frameset，这个标签现在用的已经不多了，用法比较特殊。 <http://www.w3.org/TR/REC- html40/present/frames.html#h-16.2> frameset是一个与html并列的一种文档类型，如果frameset文档中出现普通html标签，则会变成普通的html文档。
An HTML document that describes frame layout (called a frameset document) has a different makeup than an HTML document without frames. A standard document has one HEAD section and one BODY. A frameset document has a HEAD, and a FRAMESET in place of the BODY. The FRAMESET section of a document specifies the layout of views in the main user agent window. In addition, the FRAMESET section can contain a NOFRAMES element to provide alternate content for user agents that do not support frames or are configured not to display frames. Elements that might normally be placed in the BODY element must not appear before the first FRAMESET element or the FRAMESET will be ignored.