HTML elements defined by the HTML document
HTML document consists of nested HTML elements
Common elements: the start tag starting to terminate the closing tag, content element is the content between the opening and closing tags
<html></html><!--定义了整个HTML文档，是整个文档的开始到结束-->
<head></head><!--包含了所有的头部标签元素，包括文档的标题，脚本，样式，meta等-->
<title></title><!--标题-->
<body></body><!--定义文档的主体，文档的所有内容：文本，超链接，图像，表格，列表等-->Empty element
has no content of the HTML element is called empty elements Empty elements are closed in the start tag of
<br /><!--换行-->
<hr ><!--水平线-->HTML attributes
attribute extensions HTML element is provided; for the modified style tags appear as name / value pair (key-value pair)
Note: the attribute value is always included in the quotation marks, if the attribute value itself with quotation marks, then use single quotation marks, and not case sensitive
class:为html元素定义一个或多个类名
id：定义元素的唯一id
style：规定元素的内联样式（css样式）
title:规定元素的额外信息（可在工具提示中显示）
<p class=""></p>
<p id=""></p>
<p style=""></p>
<p title=""></p>
