diff --git a/source b/source
index c01f837daa4..ce7cfde5988 100644
--- a/source
+++ b/source
@@ -84869,6 +84869,23 @@ interface <dfn>NavigatorOnLine</dfn> {
 
   <h5 id="fetching-scripts">Fetching scripts</h5>
 
+  <p>The various script-fetching algorithms below have two hooks that may be customized by their
+  callers:</p>
+
+  <ul>
+
+   <li><p><dfn data-x="fetching-scripts-set-up-request">Set up the request</dfn>, which takes a
+   <span data-x="concept-request">request</span> which it may modify before the algorithm
+   continues</p></li>
+
+   <li><p><dfn data-x="fetching-scripts-validate-response">Validate the response</dfn>, which takes
+   a <span data-x="concept-response">response</span> and must either return true or false</p></li>
+
+  </ul>
+
+  <p class="note"><cite>Service Workers</cite> is an example of a specification that runs these
+  algorithms with its own options for the hooks. <ref spec=SW></p>
+
   <p>To <dfn>fetch a classic script</dfn> for a <code>script</code> element <var>element</var>,
   given a <var>url</var>, a <var>CORS setting</var>, a <var>settings object</var>, and a
   <var>character encoding</var>, run these steps. The algorithm will asynchronously complete with
@@ -84884,6 +84901,9 @@ interface <dfn>NavigatorOnLine</dfn> {
    data-x="">script</code>", and its <span data-x="concept-request-destination">destination</span>
    to "<code data-x="">script</code>".</p></li>
 
+   <li><p>If the caller specified custom steps to <span data-x="fetching-scripts-set-up-request">set
+   up the request</span>, perform them on <var>request</var>.</p></li>
+
    <!--FETCH-->
    <li><p><span data-x="concept-fetch">Fetch</span> <var>request</var>.</p></li>
 
@@ -84901,6 +84921,11 @@ interface <dfn>NavigatorOnLine</dfn> {
    data-x="concept-response-status">status</span> is not an <span>ok status</span>, asynchronously
    complete this algorithm with null, and abort these steps.</p></li>
 
+   <li><p>If the caller specified custom steps to <span
+   data-x="fetching-scripts-validate-response">validate the response</span>, perform them on
+   <var>response</var>. If they return false, complete this algorithm with null, and abort these
+   steps.</p></li>
+
    <li><p>If <var>response</var>'s <span data-x="Content-Type">Content Type metadata</span>, if
    any, specifies a character encoding, and the user agent supports that encoding, then set
    <var>character encoding</var> to that encoding (ignoring the passed-in value).</p></li>
@@ -84942,6 +84967,9 @@ interface <dfn>NavigatorOnLine</dfn> {
    data-x="concept-request-credentials-mode">credentials mode</span> is "<code
    data-x="">same-origin</code>", and whose <span>use-URL-credentials flag</span> is set.</p></li>
 
+   <li><p>If the caller specified custom steps to <span data-x="fetching-scripts-set-up-request">set
+   up the request</span>, perform them on <var>request</var>.</p></li>
+
    <!--FETCH-->
    <li><p><span data-x="concept-fetch">Fetch</span> <var>request</var>.</p></li>
 
@@ -84954,6 +84982,11 @@ interface <dfn>NavigatorOnLine</dfn> {
    data-x="concept-response-status">status</span> is not an <span>ok status</span>, asynchronously
    complete this algorithm with null, and abort these steps.</p></li>
 
+   <li><p>If the caller specified custom steps to <span
+   data-x="fetching-scripts-validate-response">validate the response</span>, perform them on
+   <var>response</var>. If they return false, complete this algorithm with null, and abort these
+   steps.</p></li>
+
    <li><p>Let <var>source text</var> be the result of <span data-x="UTF-8 decode">UTF-8
    decoding</span> <var>response</var>'s <span data-x="concept-response-body">body</span>.</p></li>
 
@@ -84972,7 +85005,11 @@ interface <dfn>NavigatorOnLine</dfn> {
    <li><p>If <var>ancestor list</var> is not given, let it be an empty list.</p></li>
 
    <li><p><span>Fetch a single module script</span> given <var>url</var>, <var>credentials
-   mode</var>, <var>destination</var>, and <var>settings object</var>.</p>
+   mode</var>, <var>destination</var>, and <var>settings object</var>. If the caller of this
+   algorithm specified custom <span data-x="fetching-scripts-set-up-request">set up the
+   request</span> or <span data-x="fetching-scripts-validate-response">validate the response</span>
+   steps, pass those along while <span data-x="fetch a single module script">fetching a single
+   module script</span>.</p>
 
    <li><p>Return from this algorithm and run the following steps when <span data-x="fetch a single
    module script">fetching a single module script</span> asynchronously completes with
@@ -84991,6 +85028,12 @@ interface <dfn>NavigatorOnLine</dfn> {
    asynchronously complete this algorithm with <var>descendants result</var>.</p></li>
   </ol>
 
+  <hr>
+
+  <p>The following algorithms are used when <span data-x="fetch a module script tree">fetching a
+  module script tree</span>, and are not meant to be used directly by other specifications (or by
+  other parts of this specification).</p>
+
   <p>To <dfn>fetch the descendants of a module script</dfn> <var>module script</var>, given a
   <var>destination</var> and an <var>ancestor list</var>, run these steps. The algorithm will
   asynchronously complete with either null (on failure) or with <var>module script</var> (on
@@ -85036,6 +85079,11 @@ interface <dfn>NavigatorOnLine</dfn> {
     data-x="concept-module-script-credentials-mode">credentials mode</span>, <var>destination</var>,
     <var>module script</var>'s <span>settings object</span>, and <var>ancestor list</var>.</p>
 
+    <p class="note">It is intentional that no custom <span
+    data-x="fetching-scripts-set-up-request">set up the request</span> or <span
+    data-x="fetching-scripts-validate-response">validate the response</span> steps are passed along
+    here. Those hooks only apply to the top-level fetch at the root of the module script tree.</p>
+
     <p>If any of the <span>fetch a module script tree</span> invocations asynchronously complete
     with null, the user agent may <span data-x="concept-fetch-terminate">terminate</span> any or
     all of the other fetches, and must then asynchronously complete this algorithm with null.</p>
@@ -85073,6 +85121,9 @@ interface <dfn>NavigatorOnLine</dfn> {
    mode</var>, and <span data-x="concept-request-client">client</span> is
    <var>settings object</var>.</p></li>
 
+   <li><p>If the caller specified custom steps to <span data-x="fetching-scripts-set-up-request">set
+   up the request</span>, perform them on <var>request</var>.</p></li>
+
    <!--FETCH-->
    <li><p><span data-x="concept-fetch">Fetch</span> <var>request</var>.</p></li>
 
@@ -85105,6 +85156,10 @@ interface <dfn>NavigatorOnLine</dfn> {
       classic script</span> does not include MIME type checking. In contrast, module scripts will
       fail to load if they are not of a correct MIME type.</p>
      </li>
+
+     <li><p>The caller specified custom steps to <span
+     data-x="fetching-scripts-validate-response">validate the response</span>, which when performed
+     on <var>response</var> return false.</p></li>
     </ul>
    </li>