Copy String context handling to attribute setter steps.

Author: lukewarlow

index.bs

@@ -6354,7 +6354,7 @@ The following extended attributes are <dfn for="extended attributes">applicable
 [{{AllowShared}}],
 [{{Clamp}}],
 [{{EnforceRange}}],
-[{{LegacyNullToEmptyString}}] and
+[{{LegacyNullToEmptyString}}], and
 [{{StringContext}}].
 
 
@@ -10280,7 +10280,7 @@ must be one of "<code>html</code>", "<code>script-url</code>" and "<code>script<
 [{{StringContext}}] extended attribute may only annotate a type of a [=regular attribute=] or
 a [=regular operation=] argument. A type annotated with the [{{StringContext}}]
 extended attribute must not appear in a [=read only=] attribute. The [=regular attribute=] or
-a [=regular operation=] argument that the type annotated with the [{{StringContext}}] extended
+[=regular operation=] argument that the type annotated with the [{{StringContext}}] extended
 attribute appears in is its <dfn>related construct</dfn>.
 
 A type that is not {{DOMString}} or {{USVString}} must not be [=extended attributes associated with|associated with=]
@@ -12041,8 +12041,14 @@ in which case they are exposed on every object that [=implements=] the interface
 
                 <dt>Otherwise</dt>
                 <dd>
-                    |idlValue| is the result of [=converted to an IDL value|converting=] |V| to an
-                    IDL value of |attribute|'s type.
+                    1.  If the conversion is to an IDL type [=extended attribute associated with|associated with=] the
+                        [{{StringContext}}] extended attribute, then set |V| to the result of [=validate the string in context=], passing
+                        [=this=], |V|, the {{StringContext}} extended attribute [=identifier=], and the [=identifier=]
+                        of the [{{StringContext}}] extended attribute [=related construct=].
+
+                        Note: That algorithm may [=JavaScript/throw=] a <l spec=ecmascript>{{TypeError}}</l>.
+                    1.  |idlValue| is the result of [=converted to an IDL value|converting=] |V| to an
+                        IDL value of |attribute|'s type.
                 </dd>
             </dl>