HTML 5中又新增加main标签
HTML 5新增加的标签元素已经够多的了，在2013年2月份，又增加了<main>标签，参考：
http://www.w3.org/html/wg/drafts/html/master/grouping-content.html#the-main-element
其中定义如下：
The main element represents the main content of the body of a document or application. The main content area consists of content that is directly related to or expands upon the central topic of a document or central functionality of an application.
Authors must not include more than one main element in a document.
Authors must not include the main element as a child of an article, aside, footer, header or nav element
也就是说，它是表达了document中的body的主要内容，但不能在一个document中有多个main,不能在article,aside,footer,header或者nav中包含main.下面是个例子：
标准中的又一个例子：<main> <h1>Skateboards</h1> The skateboard is the way cool kids get around <article> <h2>Longboards</h2> Longboards are a type of skateboard with a longer wheelbase and larger, softer wheels. ... ... </article> <article> <h2>Electric Skateboards</h2> These no longer require the propelling of the skateboard by means of the feet; rather an electric motor propels the board, fed by an electric battery. ... ... </article></main> <!-- other content -->
