Prefer `defer` Over `async`

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 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

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I'm sure you have your own opinion...