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Steve Souders:

Comparing the ASYNC and DEFER waterfalls, we see that using DEFER makes DOM Interactive fire sooner and allows rendering to proceed more quickly.

Even though <script async></script> is more commonly seen in the wild, defer is typically preferable as they can never block other syncronous scripts, as well as load in predictable source order.

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Prefer `defer` Over `async` is a post from CSS-Tricks