According to the official W3 spec, async
and defer
may be specified together, for example, like so:
<script src="myscript.js" async defer></script>
In this case, defer
will serve as a fallback in older browsers that do not support async
, while in modern browsers (that do support async
), async
will have precedence over defer
. In either case, only one of the strategies will be used.
Although, both async
and defer
download scripts in a non-blocking way, note that the former executes the script as soon as it's downloaded while the latter executes it when the page has finished parsing (before DOMContentLoaded
event is fired).
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