Incorporating some of @annevk's feedback.

From w3ctag/design-reviews#280 (comment)
and w3ctag/design-reviews#280 (comment).

index.bs

@@ -79,14 +79,14 @@ A request generated by a <{picture}> element would result in a request containin
 HTTP request header:
 
 ```
-Sec-Metadata: destination=image, site=cross-site, target=subresource
+Sec-Metadata: destination="image", site="cross-site"
 ```
 
 A top-level navigation from `https://example.com` to `https://example.com/` caused by a user's click
 on an in-page link would result in a request containing the following HTTP request header:
 
 ```
-Sec-Metadata: cause=user-activated, destination=document, site=same-origin, target=top-level
+Sec-Metadata: cause="user-activated", destination="document", site="same-origin", target="top-level"
 ```
 
 Framework {#framework}
@@ -112,10 +112,11 @@ unknown members, and members whose value is invalid.
 
 [=Navigation requests=] will contain a dictionary member whose key is
 "<dfn dict-member for="Sec-Metadata">`cause`</dfn>", and whose value is an
-<a for="structured header" grammar>`identifier`</a>. Other kinds of requests will not contain a
+<a for="structured header" grammar>`string`</a>. Other kinds of requests will not contain a
 {{Sec-Metadata/cause}} member.
 
-Valid {{Sec-Metadata/cause}} values are `user-activated` and `forced`.
+Valid {{Sec-Metadata/cause}} values are `user-activated` and `forced`, representing whether or not
+a given request was [=triggered by user activation=].
 
 <div algorithm="get cause">
 
@@ -138,9 +139,8 @@ To <dfn abstract-op local-lt="get cause">obtain the `cause` value</dfn> for a [=
 ### The `destination` Member ### {#destination-member}
 
 All requests will contain a dictionary member whose key is
-"<dfn dict-member for="Sec-Metadata">`destination`</dfn>", and whose value is either an empty
-<a for="structured header" grammar>`string`</a>, or an
-<a for="structured header" grammar>`identifier`</a> representing the [=request=]'s
+"<dfn dict-member for="Sec-Metadata">`destination`</dfn>", and whose value is a
+<a for="structured header" grammar>`string`</a> representing the [=request=]'s
 [=request/destination=].
 
 Valid {{Sec-Metadata/destination}} values include all valid [=request=] [=request/destination=]
@@ -158,20 +158,20 @@ values (the empty string, "`audio`", "`audioworklet`", "`document`", "`embed`",
 Sec-Metadata: ..., destination="", ...
 
 // `<img>`'s destination is "image"
-Sec-Metadata: ..., destination=image, ...
+Sec-Metadata: ..., destination="image", ...
 
 // `new Worker()`'s destination is "worker"
-Sec-Metadata: ..., destination=worker, ...
+Sec-Metadata: ..., destination="worker", ...
 
 // Navigations' destinations are "document"
-Sec-Metadata: ..., destination=document, ...
+Sec-Metadata: ..., destination="document", ...
 </pre>
 
 ### The `site` Member ### {#site-member}
 
 All requests will contain a dictionary member whose key is
 "<dfn dict-member for="Sec-Metadata">`site`</dfn>", and whose value is an
-<a for="structured header" grammar>`identifier`</a> representing the relationship between the
+<a for="structured header" grammar>`string`</a> representing the relationship between the
 [=request=]'s [=request/origin=] and the [=url/origins=] for the [=URLs=] in the [=request=]'s
 [=request/url list=].
 
@@ -196,26 +196,29 @@ Valid {{Sec-Metadata/site}} values are `same-origin`, `same-site`, and `cross-si
 
 ### The `target` Member ### {#target-member}
 
-All requests will contain a dictionary member whose key is
+[=Navigation requests=] will contain a dictionary member whose key is
 "<dfn dict-member for="Sec-Metadata">`target`</dfn>", and whose value is an
-<a for="structured header" grammar>`identifier`</a> representing the context in which the request
-will be used.
+<a for="structured header" grammar>`string`</a> representing the context in which the request
+will be used. Other kinds of requests will not contain a {{Sec-Metadata/target}} member.
 
 Valid {{Sec-Metadata/target}} values are `top-level`, `nested`, and `subresource`.
 
 <div algorithm="get target">
 
 To <dfn abstract-op local-lt="get target">obtain the `target` value</dfn> for a [=request=] (|r|),
-return the value corresponding to the first matching statement below:
+run the steps below:
+
+1.  If |r| is a [=navigation request=]:
+
+    1.  If |r|'s [=request/reserved client=] is an [=environment=] whose [=target browsing context=]
+        is a [=top-level browsing context=], return `top-level`.
 
-:   |r|'s [=request/reserved client=] is an [=environment=] whose [=target browsing context=] is a
-    [=top-level browsing context=]
-::  `top-level`
-:   |r|'s [=request/reserved client=] is an [=environment=] whose [=target browsing context=] is a
-    [=nested browsing context=]
-::  `nested`
-:   Otherwise
-::  `subresource`
+    2.  Assert: |r|'s [=request/reserved client=] is an [=environment=] whose [=target browsing
+        context=] is a [=nested browsing context=].
+
+    3.  Return `nested`.
+
+2.  Return `null`.
 
 </div>
 
@@ -233,20 +236,20 @@ To <dfn abstract-op>set the `Sec-Metadata` header for a request</dfn>, given a [
         1.  Append an item to |header| whose key is {{Sec-Metadata/cause}}, and whose value is the
             result of [$get cause|obtaining the cause value$] for |r|.
 
+        2.  Append an item to |header| whose key is {{Sec-Metadata/target}}, and whose value is the
+            result of [$get target|obtaining the target value$] for |r|.
+
     3.  Append an item to |header| whose key is {{Sec-Metadata/destination}}, and whose value is the
         result of [$get destination|obtaining the destination value$] for |r|.
 
     4.  Append an item to |header| whose key is {{Sec-Metadata/site}}, and whose value is the result
         of [$get site|obtaining the site value$] for |r|.
 
-    5.  Append an item to |header| whose key is {{Sec-Metadata/target}}, and whose value is the
-        result of [$get target|obtaining the target value$] for |r|.
-
-    6.  Let |value| be the result of serializing |header|.
+    5.  Let |value| be the result of serializing |header|.
 
         ISSUE(httpwg/http-extensions#627): I don't see an actual serialization algorithm in [[!I-D.ietf-httpbis-header-structure]].
 
-    7.  [=header list/Set=] &#96;<a http-header>`Sec-Metadata`</a>&#96;/|value| in |r|'s [=request/header list=].
+    6.  [=header list/Set=] &#96;<a http-header>`Sec-Metadata`</a>&#96;/|value| in |r|'s [=request/header list=].
 </ol>
 </div>