By default, javascript is loaded synchronously, that is, javascript is blocked when loading. The subsequent elements must wait for javascript to be loaded before they can be loaded again. For some javascript that is not very meaningful, if it is placed at the head of the page, it will cause If it loads slowly, it will seriously affect the user experience.
(1) defer, only supports IE
Definition and usage of defer attribute
defer attribute specifies whether to delay script execution until the page is loaded.
Some javascript scripts use the document.write method to create the current document content, but other scripts may not.
If your script does not change the content of the document, you can add the defer attribute to the
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