Suggest a rootMargin strategy for lazy-loaded elements

See #5408 (comment)

Fixes #5408.

source

@@ -6981,14 +6981,16 @@ 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>" → an
-      <span>implementation-defined</span> value ]»</p>
-
-      <p class="XXX">See <a
-      href="https://github.com/whatwg/html/issues/5408">issue #5408</a>.</p>
+      following dictionary members: «[ "<code data-x="">rootMargin</code>" → <span>lazy load root
+      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
+      margin. See issue <a
+      href="https://github.com/w3c/IntersectionObserver/issues/428">w3c/IntersectionObserver#428</a>.</p>
      </li>
     </ul>
    </li>
@@ -7026,6 +7028,29 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
    </li>
   </ol>
 
+  <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>
+
+  <ul>
+   <li><p>The value is greater than or equal to 1250px.</p></li>
+
+   <li><p>The typical scrolling speed: increase the value for devices with faster typical scrolling
+   speeds.</p></li>
+
+   <li><p>The current scrolling speed or momentum: the UA can attempt to predict where the scrolling
+   will likely stop, and adjust the value accordingly.</p></li>
+
+   <li><p>The network quality: increase the value for slow or high-latency connections.</p></li>
+
+   <li><p>User preferences can influence the value.</p></li>
+  </ul>
+
+  <p class="note">It is important <span class="no-backref" data-x="tracking vector">for
+  privacy</span> that the <span>lazy load root 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>
 
   <h3 split-filename="common-dom-interfaces">Common DOM interfaces</h3>