Allow delaying autoplay until video elements become visible

Starting autoplay when media elements loaded is too agressive. Added
supplementary saying that the user agent may delay autoplay until video
elements become visible.

source

@@ -32463,7 +32463,9 @@ interface <dfn>HTMLMediaElement</dfn> : <span>HTMLElement</span> {
       has an <code data-x="attr-media-autoplay">autoplay</code> attribute specified, and the
       <span>media element</span>'s <span>node document</span>'s <span>active sandboxing flag set</span>
       does not have the <span>sandboxed automatic features browsing context flag</span> set, then
-      the user agent may also run the following substeps:</p>
+      the user agent may also run the following substeps immediately or wait until the
+      <span>media element</span> becomes visible in the user <span>viewport</span> if it is a
+      <code>video</code> element:</p>
 
       <ol>
 
@@ -119272,6 +119274,7 @@ INSERT INTERFACES HERE
   Yonathan Randolph,
   Yuzo Fujishima,
   Zhenbin Xu,
+  Zhiqiang Zhang,
   Zoltan Herczeg,
   and
   &Oslash;istein E. Andersen,