async means the file will execute as soon as it's loaded, which can be during or after the parsing process, and thus the order in which asynchronous scripts are executed cannot be guaranteed.

<script type="text/javascript" src="script.js" async>

Moreover, modern browsers will continue to scan HTML while it's blocked and "look ahead" to see what external resources are defined further on, then load them. How browsers do this varies, so it's unreliable to expect them to behave in a specific way. To mark a resource as important and thus more likely to be loaded early in the rendering process, you can use the link tag with rel="preload".

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