Clarification on `async` and `defer` together

Not entirely sure if this is the right reference doc:
http://www.w3.org/TR/html-markup/script.html – but it doesn't make it clear
what should happen if both `async` and `defer` attributes are set on a
`<script>` element:

<script src="myscript.js" async defer>

Browsers all seem to give `async` precedence, and I agree that this is the
most desirable behaviour, however logically `async` and `defer` aren't
necessarily mutually exclusive. If both were applied, it might be
reasonable to assume the behaviour might match that of `defer` alone.

Could a note be added to this / any other relevant docs highlighting this
behaviour?

The WHATWG spec [1] suggests this behaviour, but isn't incredibly clear
about it either.


[1]
http://www.whatwg.org/specs/web-apps/current-work/multipage/scripting-1.html