defer
HTML defer Attribute （HTML延迟属性）
The HTML defer attribute specifies that a script is executed when a page has finished the parsing. It is a boolean attribute. This attribute works for external scripts and must be used only when the src attribute is present. （HTML延迟属性指定在页面完成解析时执行脚本。它是一个布尔属性。此属性适用于外部脚本，只能在存在src属性时使用。）
There isn’t any difference between HTML 4.01 and HTML5. In XHTML, the defer attribute must be specified as <script defer=”defer”> as attribute minimization is forbidden.
You can use this attribute on the <script> element.
An external script can be executed in the following ways: （外部脚本可以通过以下方式执行：）
When async is present, the script will be executed asynchronously while the page continues the parsing. （-当异步存在时，脚本将异步执行，同时页面继续解析。）
When async is not present but defer is present, the script will be executed when the page finishes the parsing. （-当async不存在但defer存在时，脚本将在页面完成解析时执行。）
When neither async or defer is present, the script will be executed immediately before the browser continues the parsing. （-当不存在异步或延迟时，脚本将在浏览器继续解析之前立即执行。）
Syntax
Syntax （语法）
<script src="example.js" defer></script>
Example of the HTML defer attribute:
<!DOCTYPE html>
<html>
<head>
<title>Title of the documnet</title>
<script src="example.js" defer></script>
<noscript>Sorry, your browser doesn't support JavaScript!</noscript>
</head>
<body>
<h1>Example</h1>
<p>The "defer" attribute specifies that a script is executed when a page has finished the parsing. It is a boolean attribute.</p>
</body>
</html>