A script that will be run asynchronously as soon as it is available:
The async attribute is supported in Internet Explorer 10, Firefox, Opera, Chrome, and Safari.
Note: The async attribute of the <script> tag is not supported in Internet Explorer 9 and earlier versions.
The async attribute is a boolean attribute.
When present, it specifies that the script will be executed asynchronously as soon as it is available.
Note: The async attribute is only for external scripts (and should only be used if the src attribute is present).
Note: There are several ways an external script can be executed:
The async attribute is new in HTML5.
In XHTML, attribute minimization is forbidden, and the async attribute must be defined as <script async="async">.
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