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：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<title>Using main</title>
</head>
<body>
<header>My page</header>
<nav>
[url=#]Home[/url]
</nav>
<main>
<article>
<h1>My article</h1>
Content
</article>
<aside>
More information
</aside>
</main>
<footer>Copyright 2013</footer>
</body>
</html>
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 -->
