Re: [heycam/webidl] Clarifications to dictionary semantics (#859)

@domenic commented on this pull request.



>  
 <p class="note">
-    In the ECMAScript binding, a value of <emu-val>undefined</emu-val> is treated as
-    [=not present=], or will trigger the [=dictionary member/default value=] where applicable.
+    In the ECMAScript binding, a value of <emu-val>undefined</emu-val> for the property
+    corresponding to a [=dictionary member=] is treated the same as omitting that property. Thus, it
+    will cause an error if the member is [=dictionary member/required=], or will trigger the
+    [=dictionary member/default value=] if one is present, or will result in no [=map/entry=]
+    existing in the dictionary value otherwise.
+</p>
+
+<p class="advisement">
+    As with [=optional argument/default value|operation argument default values=], it is strongly
+    encouraged not to use <emu-val>true</emu-val> as the [=dictionary member/default value=] for
+    {{boolean}}-typed [=dictionary members=], as this can be confusing for authors who might
+    otherwise expect the default conversion of <emu-val>undefined</emu-val> to be used (i.e.,
+    <emu-val>false</emu-val>).

Let's do that in a followup if desired, for both dictionaries and boolean arguments in general.

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