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

@domenic commented on this pull request.



> -<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. It's just a should saying not to add pointless stuff. It's not likely to be easily enforceable on an implementation level because it would require whole-corpus analysis to find all the dictionary usage sites.

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