Use scrollMargin for lazy load intersection observer

Currently the lazy load intersection observer is set up using a rootMargin.
This does not work well for pages that use images in CSS scrollers, like
carousels, causing the images to load too late, when they become visible
and the intersection with the root is detected. To work around this issue
some developers are choosing to not use lazy loading, thus increasing the
overall bandwidth usage for their sites.

Using scrollMargin for the lazy load intersection observer will allow lazy
loading images to load when they are near the viewport as expected.

source

@@ -7629,14 +7629,14 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
 
      <li>
       <p>The <var>options</var> is an <code>IntersectionObserverInit</code> dictionary with the
-      following dictionary members: «[ "<code data-x="">rootMargin</code>" → <span>lazy load root
+      following dictionary members: «[ "<code data-x="">scrollMargin</code>" → <span>lazy load scroll
       margin</span> ]»</p>
 
       <p class="note">This allows for fetching the image during scrolling, when it does not yet —
       but is about to — intersect the viewport.</p>
 
-      <p class="XXX">The <span>lazy load root margin</span> suggestions imply dynamic changes to the
-      value, but the <code>IntersectionObserver</code> API does not support changing the root
+      <p class="XXX">The <span>lazy load scroll margin</span> suggestions imply dynamic changes to the
+      value, but the <code>IntersectionObserver</code> API does not support changing the scroll
       margin. See issue <a
       href="https://github.com/w3c/IntersectionObserver/issues/428">w3c/IntersectionObserver#428</a>.</p>
      </li>
@@ -7678,8 +7678,8 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
 
   <p>
   <!--INSERT TRACKING-->
-  The <dfn>lazy load root margin</dfn> is an <span>implementation-defined</span> value, but with
-  the following suggestions to consider:</p>
+  The <dfn id="lazy-load-root-margin">lazy load scroll margin</dfn> is an
+  <span>implementation-defined</span> value, but with the following suggestions to consider:</p>
 
   <ul>
    <li><p>Set a minimum value that most often results in the resources being loaded before they
@@ -7697,7 +7697,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
   </ul>
 
   <p class="note">It is important <span data-x="tracking vector">for privacy</span> that the
-  <span>lazy load root margin</span> not leak additional information. For example, the typical
+  <span>lazy load scroll margin</span> not leak additional information. For example, the typical
   scrolling speed on the current device could be imprecise so as to not introduce a new
   fingerprinting vector.</p>
 
@@ -141466,6 +141466,7 @@ INSERT INTERFACES HERE
   Tony Ross,
   Tooru Fujisawa,
   Toru Kobayashi,
+  Traian Captan,
   Travis Leithead,
   Trevor Rowbotham,
   Trevor Saunders,