Update index.html

Co-authored-by: Marcos Cáceres <marcos@marcosc.com>

index.html

@@ -839,7 +839,7 @@ <h2>
           not [=Document/fully active=], then return <a>a promise rejected
           with</a> an {{"AbortError"}} {{DOMException}}.
           </li>
-          <li>If |document|'s [=Document/visibility state=] is not `"visible"`,
+          <li class="addition proposed">If |document|'s [=Document/visibility state=] is not `"visible"`,
           then return <a>a promise rejected with</a> an {{"AbortError"}}
           {{DOMException}}.
           <li>