Re: [whatwg/webidl] Allow DOMException subclasses to be used as exceptions (PR #1211)

@annevk approved this pull request.

Looks good modulo nits and question.

>      </blockquote>
+
+    <p>Such additional context is most helpful to implementers when it is not immediately obvious
+    why the exception is being thrown, e.g., because there are many different steps in the algorithm
+    which throw a "{{SyntaxError}}" {{DOMException}}. In contrast, if your specification throws a
+    "{{NotAllowedError}}" {{DOMException}} immediately after checking if the user has provided
+    permission to use a given feature, it's fairly obvious what sort of message the implementation
+    should construct, and so specifying it is not necessary.

```suggestion
    ought to construct, and so specifying it is not necessary.
```

> @@ -5176,6 +5197,93 @@ over just using {{SyntaxError!!exception}} to refer to the {{DOMException}}. [[D
     </tbody>
 </table>
 
+<h4 id="idl-DOMException-derived-interfaces">{{DOMException}} derived interfaces</h4>
+
+When an exception needs to carry additional programmatically-introspectable information, beyond what
+can be provided with a {{DOMException}}'s [=DOMException/name=], specification authors can create an
+[=interface=] which [=interface/inherits=] from {{DOMException}}. Such interfaces need to follow
+certain rules, in order to have a predictable shape for developers. Specifically:
+
+*   The [=identifier=] of the [=interface=] must end with <code>Error</code>, and must not be any
+    of the names in the <a><code>DOMException</code> names table</a>.
+*   The [=interface=] must have a [=constructor operation=] which sets the instance's
+    [=DOMException/name=] to the interface's [=identifier=].
+*   Their [=constructor operation=] must take as its first parameter an [=optional argument|optional=]
+    {{DOMString}} named |message| defaulting to the empty string, and must set the instance's
+    [=DOMException/message=] to |message|.
+*   Their [=constructor operation=] should take as its second parameter a [=dictionary=] containing

Is the switch from must to should intentional?

>  
 *   <dfn exception>EvalError</dfn>
 *   <dfn exception>RangeError</dfn>
 *   <dfn exception>ReferenceError</dfn>
 *   <dfn exception>TypeError</dfn>
 *   <dfn exception>URIError</dfn>
 
-These correspond to all of the ECMAScript [=ECMAScript/error objects=]
-(apart from <l spec=ecmascript>{{SyntaxError}}</l> and {{Error}},
-which are deliberately omitted as they are reserved for use
-by the ECMAScript parser and by authors, respectively).
-The meaning of each [=simple exception=] matches
-its corresponding error object in the
-ECMAScript specification.
+These correspond to all of the ECMAScript [=ECMAScript/error objects=] (apart from
+<l spec=ecmascript>{{SyntaxError}}</l> and <l spec=ecmascript>{{Error}}</l>, which are deliberately

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