Re: [dom] Add EventListenerOptions and passive event listeners (#82)

> @@ -816,7 +818,14 @@ must return the values they were initialized to.
>  The
>  <dfn method for=Event>preventDefault()</dfn>
>  method must set the <a>canceled flag</a> if the
> -{{Event/cancelable}} attribute value is true.
> +{{Event/cancelable}} attribute value is true and
> +the <a>in passive listener flag</a> is unset.
> +
> +<p class="note no-backref">
> + User agents are encouraged to generate a console warning or other debugging
> + aid to help authors identify places where calls to {{preventDefault()}}
> + have no effect.

@domenic started using "User agents are encouraged to log the precise cause in a developer console, to aid debugging." in HTML. I suggest we start from there.

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