Allow not executing scripts inserted by document.write()

This allows user agents to experiment with better heuristics for not executing such scripts, as per WICG/interventions#17.

source

@@ -57390,7 +57390,8 @@ interface <dfn>HTMLScriptElement</dfn> : <span>HTMLElement</span> {
 <!--TOPIC:HTML-->
 
   <p class="note">When inserted using the <code data-x="dom-document-write">document.write()</code>
-  method, <code>script</code> elements execute (typically blocking further script execution or HTML parsing), but when inserted using
+  method, <code>script</code> elements <a href="#document-written-scripts-intervention">usually</a>
+  execute (typically blocking further script execution or HTML parsing). When inserted using the
   <code data-x="dom-innerHTML">innerHTML</code> and <code data-x="dom-outerHTML">outerHTML</code>
   attributes, they do not execute at all.</p>
 
@@ -90111,8 +90112,10 @@ document.body.appendChild(frame)</pre>
     "<code data-x="">&lt;plaintext&gt;</code>" or "<code data-x="">&lt;!--</code>"). In other cases,
     the call can clear the current page first, as if <code
     data-x="dom-document-open">document.open()</code> had been called. In yet more cases, the method
-    is simply ignored, or throws an exception. To make matters worse, the exact behavior of this
-    method can in some cases be dependent on network latency<!--
+    is simply ignored, or throws an exception. Users agents are <a
+    href="#document-written-scripts-intervention">explicitly allowed to avoid executing
+    <code>script</code> elements inserted via this method</a>. And to make matters even worse, the
+    exact behavior of this method can in some cases be dependent on network latency<!--
 
      Namely, in the following case:
 
@@ -104340,6 +104343,14 @@ document.body.appendChild(text);
      algorithm</span>, then mark the <code>script</code> element as <span>"already started"</span>.
      (<span>fragment case</span>)</p></li>
 
+     <li id="document-written-scripts-intervention"><p>If the parser was invoked via the <code
+     data-x="dom-document-write">document.write()</code> or <code
+     data-x="dom-document-writeln">document.writeln()</code> methods, then optionally mark the
+     <code>script</code> element as <span>"already started"</span>. (For example, the user agent
+     might use this clause to prevent execution of <span data-x="origin">cross-origin</span> scripts
+     inserted via <code data-x="dom-document-write">document.write()</code> under slow network
+     conditions, or when the page has already taken a long time to load.)</p></li>
+
      <li><p>Insert the newly created element at the <var>adjusted insertion
      location</var>.</p></li>
 
@@ -119763,6 +119774,7 @@ INSERT INTERFACES HERE
   Keith Yeung,
   Kelly Ford,
   Kelly Norton,
+  Kenji Baheux,
   Kevin Benson,
   Kevin Gadd,
   Kevin Cole,
@@ -120016,6 +120028,7 @@ INSERT INTERFACES HERE
   Shaun Inman,
   Shiino Yuki, <!-- yuki3 on GitHub -->
   Shiki Okasaka,
+  Shivani Sharma,
   Shubheksha Jalan,
   Sierk Bornemann,
   Sigbj&oslash;rn Finne,