A script that will be run asynchronously as soon as it is available:
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 numbers in the table specify the first browser version that fully supports the attribute.
The async attribute is new in HTML5.
In XHTML, attribute minimization is forbidden, and the async attribute must be defined as <script async="async">.