Define srcdoc documents as matching about:srcdoc URL

Previously, the standard defined "an iframe srcdoc document" as being solely constructed from the contents of an iframe's srcdoc content attribute. However, it is possible to create documents that are not created in such a way, but do have a URL of about:srcdoc (or about:srcdoc#foo), using document.open() and javascript: URL navigation.

Remove the distinction between such nontraditional srcdoc documents and the classic "iframe srcdoc documents" by introducing the "matches about:srcdoc" concept for URLs, and defining "an iframe srcdoc document" as a thin alias over this concept. This proposal was described by #9514 (comment). This matches observed browser behavior, for referrer calculation (the only post-parsing case that is impacted by the "an iframe srcdoc document" concept).

Additionally, this new concept ensures that about:srcdoc#foo documents (e.g., created via history.replaceState(null, '', 'about:srcdoc#foo')) are also counted as normal srcdoc documents.

Fixes #9514.

source

@@ -6988,6 +6988,8 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
    <span>tree order</span>.</p></li>
   </ol>
 
+  <hr>
+
   <p>A <span>URL</span> <dfn>matches <code>about:blank</code></dfn> if its <span
   data-x="concept-url-scheme">scheme</span> is "<code data-x="">about</code>", its <span
   data-x="concept-url-path">path</span> contains a single string "<code data-x="">blank</code>", its
@@ -7000,6 +7002,25 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
   record</span> created by <span data-x="URL parser">parsing</span> "<code
   data-x="">about:blank?foo#bar</code>" <span>matches <code>about:blank</code></span>.</p>
 
+  <p>A <span>URL</span> <dfn>matches <code>about:srcdoc</code></dfn> if its <span
+  data-x="concept-url-scheme">scheme</span> is "<code data-x="">about</code>", its <span
+  data-x="concept-url-path">path</span> contains a single string "<code data-x="">srcdoc</code>",
+  its <span data-x="concept-url-query">query</span> is null, its <span
+  data-x="concept-url-username">username</span> and <span
+  data-x="concept-url-password">password</span> are the empty string, and its <span
+  data-x="concept-url-host">host</span> is null.</p>
+
+  <p class="note">The reason that <span>matches <code>about:srcdoc</code></span> ensures that the
+  <span>URL</span>'s <span data-x="concept-url-query">query</span> is null is because it is not
+  possible to create <span data-x="an iframe srcdoc document">an <code>iframe</code> <code
+  data-x="attr-iframe-srcdoc">srcdoc</code> document</span> whose <span
+  data-x="concept-document-url">URL</span> has a non-null <span
+  data-x="concept-url-query">query</span>, unlike <code>Document</code>s whose <span
+  data-x="concept-document-url">URL</span> <span>matches <code>about:blank</code></span>. In other
+  words, the set of all <span>URL</span>s that <span data-x="matches about:srcdoc">match
+  <code>about:srcdoc</code></span> only vary in their <span
+  data-x="concept-url-fragment">fragment</span>.</p>
+
 
   <div w-nodev>
 
@@ -32541,8 +32562,10 @@ interface <dfn interface>HTMLIFrameElement</dfn> : <span>HTMLElement</span> {
 
   <p>The <dfn for="iframe" element-attr><code data-x="attr-iframe-srcdoc">srcdoc</code></dfn>
   attribute gives the content of the page that the element's <span>content navigable</span> is to
-  contain. The value of the attribute is the source of <dfn export>an <code>iframe</code> <code
-  data-x="attr-iframe-srcdoc">srcdoc</code> document</dfn>.</p>
+  contain. The value of the attribute is used to <span data-x="create navigation params from a
+  srcdoc resource">construct</span> <dfn export>an <code>iframe</code> <code
+  data-x="attr-iframe-srcdoc">srcdoc</code> document</dfn>, which is a <code>Document</code> whose
+  <span data-x="concept-document-url">URL</span> <span>matches <code>about:srcdoc</code></span>.</p>
 
   <p>The <code data-x="attr-iframe-srcdoc">srcdoc</code> attribute, if present, must have a value
   using <span>the HTML syntax</span> that consists of the following syntactic components, in the