[attribute comparison] defer and async

• defer

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  defer Wait until the entire page is rendered normally in memory （DOM The structure is completely generated , And other script execution completed ）, Will execute ;

• async

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  async Once the download is complete , The rendering engine will interrupt rendering , After executing this script , Continue rendering .

• summary

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  In a word ,defer yes “ After rendering, execute ”,async yes “ Download and execute ”. in addition , If there are more than one defer Script , They will be loaded in the order they appear on the page , The multiple async Scripts don't guarantee the loading order .