defer是指js中的代码不是需要立即执行的。Using the attribute at design time can improve the download performance of a page because the browser does not need to parse and execute the script and can continue downloading and parsing the page instead.
defer [CI] When set, this boolean attribute provides a hint to the user agent that the script is not going to generate any document content (e.g., no "document.write" in javascript) and thus, the user agent can continue parsing and rendering.
When set, this boolean attribute provides a hint to the user agent that the script is not going to generate any document content (e.g., no "document.write" in javascript) and thus, the user agent can continue parsing and rendering.