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

@annevk approved this pull request.

I think this looks okay, but @EdgarChen / @TimothyGu should probably do a final check.

> -<dfn id="dfn-present" export lt="present|not present" for="dictionary member">present</dfn>
-in a dictionary value if the value [=map/exists|contains an entry with the key=]
-given by the member's [=identifier=], otherwise it is [=not present=].
-Dictionary members can also optionally have a <dfn id="dfn-dictionary-member-default-value" for="dictionary member" export>default value</dfn>, which is
-the value to use for the dictionary member when passing a value to a
-[=platform object=] that does
-not have a specified value.  Dictionary members with default values are
-always considered to be present.
+[=Dictionary members=] can be specified as
+<dfn id="required-dictionary-member" export for="dictionary member">required</dfn>, meaning that
+converting a language-specific value to a dictionary requires providing a value for that member.
+They can also be specified as having a
+<dfn id="dfn-dictionary-member-default-value" for="dictionary member" export>default value</dfn>,
+which is the value used by default when author code or specification text does not provide a value
+for that member. Note that [=dictionary member/required=] dictionary members only have meaning for
+dictionaries used as operation arguments; members should be left optional if a dictionary is only

This is new, right? Why is this a should and are implementers on board with enforcing this so we can ensure it doesn't end up in specifications?

>  
 <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
+    suggested not to use <emu-val>true</emu-val> as the [=dictionary member/default value=] for

```suggestion
    encouraged not to use <emu-val>true</emu-val> as the [=dictionary member/default value=] for
```

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